How can I stop the boiler from blowing the solenoid valves?

I have a Glow worm Hide Away 120B boiler. We have had no heat or hot water due to solenoid not working. We have had an engineer in to look at the boiler and he does not know what is causing the problem. We have now replaced 3 gas valves(cheaper than the solenoids) , each time the boiler jumps to life, we feel heat fro a few mins and then it completely stops again. Any advice?