Ihave a 1994 Rheem furnace it is a 75,000 BTU with a hot surface ignition. Once it is triggered, the ignition takes place but shuts itself off after 5-7 seconds thus blowing out cool air. I have tried reversing the plug to make sure it is correctly plugged into the wall (polar). Once it goes through the heating cycle, since it hasn't heated the house it comes on again immediately does the cycle again (gas ignited only a few seconds)then shuts off for 15 minutes and we go through the 2 cycles again. It works fine about once per 20 ignitions. I am stymied.

Could be the HSI glow bar. Is there a small purge fan motor mounted in the front of the furnace? If so many times there is a switch/vacuum switch that could be bad or the plastic tube going from it to the furnace could be clogged with debris.