I have a Bryant 394GAD that runs fine for days, then begins to have issues with the Vent Shut Safety Switch tripping and shutting down the flame. The fan continues to run and blow cold air throughout the house. If I reset the switch, the burners will come back on and the furnace will run. Sometimes it will not run unless I let the system sit for a about an hour or two before turning it back on. When this happens, I simply let the system sit for a few hours before restarting it and it will usually run for another day or two before the problem occurs again. For some reason, it seem that the problem occurs only at night. We did not experience any issues when running the AC in the summer time. Do I have a bad switch? Sensor? CKT Board? Should I just give up and buy a new furnace?

Check the flue/furnace discharge tube for restriction.
Could be a weak switch.