I feel your pain, been there myself. This could be a problem with your boiler, it could be your system is gunked up, or it could be your thermostat commanding the heating system to turn on and off. My money would be the boiler and not the thermostat, but you could take the communication wires out of the thermostat and twist them together for awhile to see if your boiler stays on. If it does then buy a new thermostat and count yourself enormously lucky that it was that simple and cheap.
It could be that your heating system needs a flush, which is something you could do yourself with basic tools and some hose. Generally you would flush the system with water, then refill and add a cleaning compound for a couple of weeks, and then flush again and refill being sure to add a conditioner. If you live in the UK then these instructions may work for you. If you're not sure what you're doing then get a professional.
If it is your boiler and not your thermostat then your best option is to get in a very good heating engineer who really knows your boiler type well. There are simply too many boiler types and things that could be going wrong to give you advice, and you probably wouldn't be able to fix it on your own anyway.
On page 5 of the boiler manual you linked to, it states:
PRE-HEAT DHW pre-heat will be disabled during DHW off periods.
So, you need to define some DHW "off" periods (most of the time, evidently) and a few DHW "on" periods when you expect to use it.
Programming those is described on Page 8.
The default is to run DHW "on" from 06:30 to 00:00 (page 11)
Edit: I have observed that most generalizations don't hold worldwide, so I did not start out with this one - but certainly the vast majority of North American "heating and domestic hot water" (what you presumably mean by "combi" - not a term we use) boilers simply run to keep themselves hot all the time - there may be a summer/winter switch which changes some setpoints, but the basic operation is to sit there being warm/hot as a default. So yes, I'd consider that "normal" operation for such a boiler, if it hasn't got the time controls as described. And @RedGrittyBrick's suggestion is a good one, if a newer control panel is compatible with an older boiler. The other change from older boilers we commonly see here is a LOT more insulation - according to the manual linked, your hourly burn is somewhat more frequent that expected, at least on the newer model - unless your boiler is located in a particularly cold place, that may well mean the newer models are better insulated (and/or, if you have not already done so, you may want to add insulation to the water pipes.) You might ask the manufacturer if there is a way you can add more insulation to (at least some parts of) the boiler itself without causing a hazard.
We had a period (not entirely extinguished, but somewhat tempered by observed results now) when "instant" hot water heaters, where only a pilot light, or sometimes not even that, was kept burning until water is called for, were all the rage. "Observed results" include far less savings (.vs. a well-insulated tank-type hot water heater) than claimed, and relatively delicate heat exchangers and control systems which become expensive to maintain after a few years. Some folks still love the things - they still hold a certain attraction for me, but being off the natural gas grid, not much - bottled/tank-trucked gas (propane) is far a more expensive fuel source.
Best Answer
I'd suspect the NTC (negative temp coefficient) thermistors as being faulty or not in good thermal contact with whatever they're supposef to be in contact with. That's a known cause of F25, and most other causes would indeed be related to heating circulation. However circulation problems / pressure / air in the system could still cause this problem even on hot water.