I'd love to know if you ever found the solution to this, I have the same problem with the same boiler.
I have been trying a variety of approaches and so far have replaced the main PCB (twice), gas valve PCB, temperature sensor (thermister), and overheat switch. I've had 3 different engineers out to it, none of whom have cured the problem. The last one replaced the thermister after it stopped working altogether, and that worked for a while (with the boiler stat turned right up, admittedly), although checking a while later I found the thermister value of the "old" one to be identical to the new one (1500KOhm when cold) so I doubt that was it.
Anyway, it is back to it's old tricks, starts, runs about 30 secs, shuts down and immediately restarts.
The Grundfos pump is working OK and not noisy (even on speed 3), but I'm beginning to wonder if it could be flow rate that is the problem. The pipework is long and convoluted as it is a largish house and the boiler is in a cupboard above the hot water tank in a first floor bathroom.
Before I trash this 4 year old boiler (!) is there anything else I can try?
Beer is only a short term solution...
