Aircon cold enough only when weather is cold

Insulation will degrade over time. However, we're talking about decades, not months or even a few years. Even blown fiber or cellulose insulation should provide a good R-value for as long as the house is structurally sound; if you need to vacuum out and re-blow or re-lay insulation, chances are the house needs a LOT more work than that.

What is probably happening is exactly what BMitch said; keeping a place warm in the winter is easier than keeping it cool in summer. All the external heat sources such as solar gain that help you out in winter work against you in summer. Heat is also easy to come by; everything in your house that consumes electricity, natural gas, or food produces heat, by the second law of thermodynamics (no system in the universe can operate at 100% efficiency). Taking heat out of air is harder, because those same laws are now working counter to your goal, adding heat back in even to the system that is providing the cooling.

I'm from Texas, so I know a few things about heat. Here are some tips:

• The best you can do with your average residential Freon evaporative system is about 30 degrees less than the outside air. The radiator in your exterior unit can, at its very best, only reduce the temperature of the compressed coolant to that of the outside air, then when the coolant is allowed to expand into a gas in your evaporator coil, the cooling effect of the state change produceas a delta of about 30*. So, if it's 105* outside, your best efforts inside will only get you to about 75*. Knowing that up front will allow you to adjust your expectations. You can go cooler than that, but most methods will require a second "piggyback" system that uses the air cooled by your main system to cool its compressor coil, which increases your energy cost per degree.
• By the ideal gas laws, a given volume of air takes up less space and has lower pressure when cooled. This means your A/C, by taking a volume of warm air and cooling it, will give your house's interior a slightly lower air pressure than the outside, which will cause outside air to be drawn into the house anywhere it can get in. This makes a good weather seal, and keeping your doors and windows closed during the heat of the day, critical to keeping your house cool.
• Anything that outside air can touch directly will conduct heat from the outdoors in. So, make sure every exterior surface of your home is well-insulated. Double-pane insulated windows can be an expensive retrofit, but they do quite a bit to help out in both summer and winter.
• After the outside ambient air temperaure, solar gain is the next biggest source of heat gain in your house. Depending on the shade from trees around your house and the size of your windows, it may even be the primary concern. Solar screens on the outside of all windows are a must; without blocking out all light, they reduce the incoming infrared rays to drastically reduce solar gain. Horizontal blinds for windows and vertical blinds for patio and panel doors are also must-haves; in addition to blocking sunlight, they also give you a little R-value by trapping the air warmed by the windows between the window and the blind slats. Skylights look good and reduce your lighting bill, but they are designed to let in the sun, which lets in heat; consider a tint or screen. You can also make a removable radiant shade for windows you don't mind covering completely, by sandwiching a layer of aluminum foil between two layers of corrugated cardboard, with an appropriate decorative covering inside and out, then use some retaining brackets around the window frame to keep it against the glass. The foil, even behind the cardboard, will reflect heat back outside, without the tacky look of just plastering aluminum foil to the window, and the cardboard will give you a little R-value.
• Stay low. Heat rises, so your upstairs will be warmer than your downstairs even if you have dual-zone HVAC. A lot of houses down here, especially newer ones, have the master bedroom on the first floor of a two- or three-story house, primarily for this benefit (it also helps the homeowners out as they get older; they can have the same two-story house they raised their kids in, without needing to climb stairs to get to bed).
• You will be surprised how well a ceiling fan works to keep you cool even if the interior of your home is edging toward 80. The fan circulates air, which helps in two ways: it mixes the standing ait in the room with the air coming from the vents to provide a more consistent temperature throughout the room, and more importantly, the movement of air takes away the "heat bubble" that your body naturally builds around itself, and moves air against any sweat on your skin which speeds evaporation and cooling. However, it doesn't help much when you're out of the room, so only run the fans in rooms you're occupying.
• Instead of fighting the sun over every inch of your home, consider using it to help you out. Solar water heaters and full-on solar generator panels use the free energy from the sun to lower the amount of energy you need to draw from your utilities. Because they soak up this energy, they also provide a little R-value over the areas they cover. They can be expensive though, and you'll have big expensive glass panels just waiting to be damaged in the next hailstorm; make sure your homeowners' insurance will cover solar panels in these cases.
• Learn to acclimate yourself from season to season. Bumping up your thermostat to 76 vs 72 in summer, dressing cool and hydrating with cold drinks, just like setting it to 68 in the winter, boiling some tea and putting on a sweater, will save you more money than all these tips put together.

You may be looking at the problem backwards, the freezing lines could be a symptom of the problem not the cause. Start by looking for blockages in the system that would cause lower air flow / heat exchange.

• Dirty filters.
• Clogged ducts.
• Closed dampers.
• Closed/Blocked vent covers.
• Dirty coils.

Basically if the system cannot exchange the heat/cold, it will not function properly and could lead to over heating/cooling.

Frozen lines could also be caused by high or low levels of refrigerant, so you'll want to get a service person out to check the refrigerant levels.

Here is a really good image to visualize how it works.