combi boiler problem continued...

[bossychloe]

Hi, My problem is that when I turn the heating on the boiler thermostat rises in about 30 secs to 85 degrees. It then cuts out and falls back to the 'cold' level before rising quickly again and the cycle continues. As the boiler is only running in short bursts the radiators just become slightly warm and a couple are stone cold. I have discovered that when the thermostat has risen and cuts out that if I switch the heating off and straight back on again the boiler fires up and prevents the gauge from falling too low. It still cuts out after a few seconds but if I continue to do this a few times it sometimes stays on for an hour or two.
Also, I have to top up the system weekly to maintain the pressure.
It is an almost new Biasi boiler but was fitted by a totally unreliable fitter during my renovation. He was difficult enough to keep on the job then and is un-contactable now.
The heating worked fine for the first few weeks although I have always needed to 'top up.' There are no problems
with the hot water.
Does anyone have any idea as to what the problem is likely to be as I have had 3 fitters have a look at it and they don't seem to want to know. One said he'd try the pump,another said it might need a lockshield valve on a rad and the other said call the manufacturer.
Can anyone help?
Thank you.