Ravenheat 85 cycling on/off?

My ravenheat 85 a works fine when asking for HW however when the CH is demanded the burner will ignite, burn for 20 seconds or so, then extinguish.
After10 seconds,it will reignite and cycle like this maybe 8 times, then sometimes settles down (rarely now)and works normally until the room stat shuts it down.
Any ideas out there what may be the problem ?
I have measured the CH thermistor, i'm getting 9.4 k/ohms when the system is cold,so I tried a 9k/ohm resistor across the ch wires, just to prove it.The system still cycles even with the fixed resistance in place.
One other thing is if I run a hot water tap for a few seconds the burner stays lit and the CH works until the room stat shuts it down!!!!and the therm' drops down to approx 4.5 k/ohms when the piping is hot which I assume is good
Any ideas out there are welcome its doing my head in.

billthespark