It's possible that you have a bubble in the fuel
line... Make sure that the pump is delivering fuel
to the unit.
IRRC the pump speeds up once the glow plug
get hot enough to combust the fuel... so the tick of the pump is slow and regular until it "fires" and then it pumps more fuel.
could be carbonized I suppose
Check to see that your exhaust
hose is clear
Aside from this you could have a failure in the control board or even the T stat could have gone south... or a corroded electrical