Am starting my third winter with this unit. Started coming on in the morning and then not again. Tech replaced a safety related pressure switch [? ] with no change and then increased the gas input rate. That helped some so now it comes on, goes through a complete heating cycle [flame and fan come on, flame goes off and then fan goes off about 30 seconds later] in about five minutes and then immediately starts up again going through normal five minute cycles until desired heat is reached. It was suggested that my thermostat needs to be replaced, which I did, but that did not fix the problem. Now I'm being told that my thermostat is too close to the cold air return register and drawing cold air to the thermostat thus shutting it down. If this were a new furnace it might be plausible but it hasn't been a problem for the past 2 years with this furnace and has been the arrangement for at least 40 years. Besides I would think the air being drawn in would be warmer not colder as the heat is on. I plan to temporarily move the thermostat just to see but don't hold out much hope for that. He also said the plenum might be getting too hot and wants to slow the fan down. Any ideas or suggestions?