Sounds like I'm having the exact same problem. Had a back boiler replaced just over a year ago with a HeatLine and have had nothing but headaches really. The problems you're talking about started after a few weeks, and eventually the guyy who installed it came back out and fixed it....
And then came back out and fixed it.....
And then came back out and fixed it.....
But eventually it stayed working OK. Until about two days ago when I just thought I'd check the pressure on it as the overflow pipe was dripping constantly. The reading was just on 3, which I know from all the visits is not a good thing and it will cut itself out.
Released some pressure by letting some water out of a rad and dropped the reading to just on 1.
However, now we have the old problem back!!!
Hope you have better luck with yours - we're gonna try and get our plumber out again. (Got to add that our plumber is a decent bloke and has made no quarms or asked 4 additional expenses about having to keep revisiting this).
Rob
April 2009