Another thing to check is for a poor connection on the molex connector on the circuit board. Pull the plug and check their condition. I sometimes will spread the terminals a bit and after it is back together, take a pick and make sure each terminal is fully seated. Sure sounds like you have a problem in the high limit switch circuit though. When you replaced that switch, did you shine a flashlight in the hole just to be sure there isn't a crack in the exchanger where the switch sets? That would cause it to open up as well. But, it should do it every time if that was the problem.