I'd say check the flame sensor for corrosion. The flame sensor on my 1999 Janitrol (GPD series) has needed to be cleaned about twice a year since it was installed (it's a very poor design). It uses a single signal wire and grounds through the plate it mounts too. A little corrosion and no ground, so the control board sees an open circuit and closes the gas valve (cycle repeats 3 times, then a 1 hour lockout - diag light blinks once). Seems to be a very common problem.