Baxi 105e c/htg cuts out of its own accord?

No problem on hot water. C/htg does not cut in if temp lights display a high temp. To get the boiler running for c/htg I have to switch off, run a hot tap to reduce the shown temp then switch to c'htg + hot water mode. The c/htg then cuts in. If the c/htg temp control is turned down for slightly warmer weather the c/htg wil spontaneously cut out (anything from 1 to 4 times daily) but the red temp lights climb 60/80 deg after the c/htg has failed. The boiler will not reignite until the temp has been reduced as above.

When restarting as above, there is a cavitating sound from the boiler unit as though there is a lot of air in there somewhere.

The problem is more pronounced when the hot water pre-heat is on; therefore I'm running it as a straight 105 at the moment and getting overheated on warmer days (but not so warm I can turn the heating off!) Two engineers can find not fault and it's a bit embarrassing. HELP!!!!

Any suggestions greatly appreciated. Roy