My AC and Heat isnt working Where should I start looking? I have tried replacing teh thermostat, bringing down teh temprature very mow as well as taking it high Neither will force the AC or heat to kick in Any ideas ? Getting desperate... I have a AC unit from Amana and Gas based\forced ai heating I bought the house from someoen so am not teh original purchaser of these units

I have a Lennox gas furnace model G12D-82-4, manufacture date of 1/86. 82,000 btu's, in a 50 year old house about 1,000 sq ft and I have a issue I am hoping someone can help with. This started last year with the burners cycling on and off about every 30 to 45 seconds, and blow out the pilot light every once in a while. The fan/limit switch would click about 4 times, then shut the burners off. I could hit the switch and the burners would come back on for a bit. I have replaced the fan/limit switch last year, was still having the problem. I had a repairman come and look. He was suggesting the furnace was tripping the high limit and found no problems, no blockage and the heat exchanger looked ok, so he turned down the gas pressure a little and it seemed fine the rest of the heat season. When i turned it on a few days ago, it was doing the same thing, cycling, fan/limit switch clicking etc...I left it off for a few days and tried to turn it on again, only now i have no burners, but the pilot is still lit. The t-stat clicks on for heat, it has the delay of the burners, then the blower starts, but no burners. The blower will run until i flip the switch on the t-stat. Is it possible the fan/limit swtich is bad again? It has a "cam stat" and the limit is set at 200 degrees and the fan off is set at 100 degrees, it has the pre heater wires going to it too. Thanks