repair to glowworm swift flow 100?

when hot water is running the green running light goes out and also the mains amber light after a half run bath .but pilot stays alight .then after a few minutes they come back on . our heating engineer said there was a pipe that was blocked at the bottom of the boiler ,which he firstly cleared then he replaced on asecond visit which has still not solved the problem .i have been told it could be the thermiter.