Problem with my Vaillant eco tech 428?

I have got a Vaillant eco tech 428 heat only boiler which has been installed to a pressurised heating system. When I put the hot water on 'auto' at the programmer the boiler will fire up and heat up the hot water cylinder then after a while the boiler will go out once its reach temp and the pump over run will kick in. The problem is that it then fires up again a few minutes later reaches temp (roughly 65 degress) rather quickly, then goes to pump over run. This will keep happening say every 5 mins. This cant be right? I dont whats causing it to fire up so frequently, is it the boiler, three port valve, cylinder stat, by pass valve not working correctly. I know these boilers dont like air but system doesnt seem to have any air in it as its very quite and has auto air vents fitted at highest points. If anyone has any ideas or solutions to what the problem might be, please let me know as its driving me mad, just cant get me head round it. Cheers Kris