problem with my biasi riva cc impact he mk2m 96 a 24 sm/c?

I have an intermittent fault, when in HW mode the boiler fires normaly with hot tap turned on but sometimes it will not stop firing until lockout when the hot tap is turned off, I then turn the power supply off, after 2 to 3 hours I then turn the power supply back on and every thing is ok for 2to 3 months or more till next time.I'm a retired gas service engineer and I have not come across this problem before and I would be grateful if you have any suggestions.