Working on a 14 year old Clare horiziontal Megasave LPG furnace. Intermintently, flame starts and immediately "poofs" out, then may re-start and run ok. Occasionally gas will shut off for no apparent reason after unit has been running for a few minutes then cycles right back on. Incoming and manifold pressures check ok; replaced power vent motor assembly which was very hot and noisy, pressure switch replaced, and installed new step-up gas regulator. Checked intake and output vent pipes -- ok and clear. No change to problem. Have checked every system I can think of. Any ideas?