The furnace usually will fire up OK, but after a few mins. the Main Gas valve closes, then reopens after a few seconds, then will sporadically close & reopen, while the Pilot stays on and the fan continues to run the entire time. If it's not too cold out, the furnace will continue to run until it satisfies the thermostat setting. But when it gets colder at night, requiring the furnace to run longer than say 15 mins, the valve will open & close until it eventually shuts down and will not restart until I reset it with the On/Off switch. I don't have a manual and can't find any info on this particular model. It's about 20 years old. I suspect the problem is with the 3 wire safety switch module (?). Any help would be appreciated.