Potterton Profile 30E keeps tripping on Central Heating?

My Potterton Profile 30E keeps tripping off and requires the reset button to be pressed to restart. This typically happens once a day but can sometimes trip up to 2 or 3 times a day. Heating engineers have looked at the problem and failed to resolve it. At times the room stat will only trip off at 10 degrees even though the room temp is warmer (this was changes 2 years ago). Recently the boiler has had a new pcb, thermostat, gas valve and pump (cheaper to buy a new one probably!!). I also have air rushing round the system which can be heard in the radiators and hot water flowing into the cold water header tank. No amount of bleeding removes the air. The pipework is approx 25 years old and was converted to gas from an oil based system 12 years ago. Could it be sludged, blocked pipes , faulty thermopstat? No idea what is causing any of these problems but any help or advice would be appreciated.