I have a comfortmaker furnace that is vented with a 3" duct and a blower. Recently it started where the furnace will run through it's course , the glow plug will work, then the gas valve opens and you can see the heat in the exchange and then after a few minutes the blower will come on and heat will go through the house. When it reaches the right temperature the furnace will shut down like its supposed to. It will run it cycles for four to five hours and then go on lockdown. The vented blower will run but the glow plug won't start and the fan just runs. I replaced the hot surface ignition control and the furnace started right up and ran for 10 hours before it went into lockdown. When this happens you turn the thermostat to the off position and then wait five minutes and turn it back to the heat position and it starts right up. I am at a loss and already had two different service guys out and they can't figure it out. In the meantime the temperature today is going to be 8 degrees. I need help! Can it be the vacuum switch that's hooked to the blower motor?

I've had similar problem. I had a service guy out and he put a pressure gauge on the motor, pressure switch connection and it registered okay but my problem is still intermittent.