We have had an intermittent problem over the last 4 months with our boiler cutting out with F22 and F25 errors (circulation fault/air in system etc...). Its a Glow worm Flexicom 18hx and has been installed for about 2 years. It would cut out every 10 minutes or so, and the way we would get it working again would be to turn off every radiator but one, and then reset the boiler multiple times (sometimes all day) until it stayed on with the one rad on. Then we would turn another radiator on, let it get hot, then another until they were all on. 

It would then work flawlessly from then on for a few weeks. Then it would start over again with the errors, and we would go through the process again. Some plumbers we have had in for other things (while there were no problems) have said it sounds like the pump was on it's way out, and it did sound funky.

This weekend it started again and no matter what we did the boiler would not stay on past 5 or 10 minutes. So I went and bought a new Grundfos pump, a drop in replacement for my old one and replaced it myself. However even with this new pump, the boiler is still crapping out.

It's a vented system with a hot water tank in the airing cupboard. The system has been drained down, all radiators bled, all bleed points by the tank and pump have been bled. The temp on the boiler is set to 39C. I have also manually opened both Honeywell valves by the pump.

Nothing is working and it is killing. What else can I possibly try?