Having a similar problem. Just started this heating season. After several times of "fixing" it without really knowing what I did, the last time I happened to bump the pressure switch while it was running and the burners went out (I am thinking the pressure switch has to stay activated while unit is running burners, is that a daaaaa?) I decided to pull the connectors at the pressure switch a couple of times (I've seen service techs do that during their $300 seasonal checkups) and so far the unit is cycling OK.

Can someone tell me what the Conversion Kit 325878-751 mentioned in this set of answers is and where to get one?