while on the aristons boiler keeps on going off the pilot light goes off nothing works...when i turn on the tap the boiler tries to get on but fails to turn on..when i bang it then the pilot light turns on..then everything starts working then after 2 minutes the pilot light switches off then again we have to bang it and it starts again i want to know why this happens what is the problem and how it can be fixed can soneone please help me...if someone can explain this with a photo we will be grateful