boiler not working properly?

we have a ravenheat combi rsf 100et. the problem is the heating fires up then goes off you have to keep switching it on and off till it works (red light keeps coming on ) sometimes this works sometimes not. then when you turn hot water tap on heating cuts out again, have same problem when we have just the water on keeps cutting out end up with no heating or hot water. have heard that it might be the htg sensor but can't find it in our manual. there is only a d.h.m/ch sensor. is this the same thing ?
many thanks

simon faulkner
November 2008