When calculating the minimum size of a range hood, there are three things you should consider. The size of the cooking surface, the amount of heat produced by the cooking surface, and the volume of the kitchen.
If the range hood is attached to a wall, you should have 100 cubic feet per minute(cfm) per linear foot. So if you have a 30" wide range, you should have a hood rated at 250 cfm ((30/12)*100 =250). If the hood is over an island, you'll use 150 cfm/linear foot. In this case that same 30" cook top, would require 375 cfm ((30/12)*150 = 375).
Next we'll determine the minimum capacity based on British thermal units(BTU)/hour, by dividing the BTU/hour by 100. For example, if we had a cooktop that produced 40,000 BTUs, we would need 400 cfm. If you are using an electric range (measured in watts), simply multiply watts by 3.41214163 to determine BTU/hr.
The final calculation, will be based on the size of the kitchen. The air in the kitchen should be cycled 15 times per hour, so our formula will be ft³/4. If we have a 10ft x 10ft x 8ft kitchen, (10 X 10 X 8)/4 = 200 cfm.
We'll then choose the largest from these three calculations, and that will be the minimum size hood we need. If you are doing more cooking than the average person, or just want a little more air movement. You can always get a larger hood, this is just the minimum size you should consider.
International Residential Code (IRC), says the minimum intermittent exhaust rate for a kitchen is 100 cfm, while the minimum continuous exhaust rate is 25 cfm.
M1507.4 Local exhaust rates. Local exhaust systems shall be designed to have the capacity to exhaust the minimum air flow rate
determined in accordance with Table M1507.4.
So you'll want to make sure the hood is at least capable of achieving these flow rates.
I'll bet the control switch is broken. This is not a difficult fix, but usually requires lifting the range top to access the wiring harness and install a new switch. Most appliance stores with a service department will also sell the parts. If your range is a new solid state computer controlled, you may have a bad control board. If that is the case, the parts are expensive and should be done by a tech. they are very hard for a consumer to trouble shoot.
Best Answer
They can indeed wear out, though it's much more exciting than what you describe - when the element in my oven (different location, same tech, 40+ years old) blew, it made a lovely arc, and I've heard people describe the same alarming thing happing on the stovetop, sometimes burning a hole in a pot.
I think what you are describing sounds more like the connectors that the elements plug into being dirty, worn, or generally connecting poorly - and once they start doing that, the connector heats up more, and that makes it more prone to corrode more, relaxes the springs, etc. making the problem worse.
There could also be a problem with the "proportional controller" which is what the setting dial adjusts - essentially a timer that sets the element on all the time on high, and turns it on and off when on lower settings, with longer off times and shorter on times at lower settings.