I changed the thermostat to see if that was the problem but it was not.. It whould start up just like normal but it seems after it gets to a certian point ( getting Hot) it would just shut itself down. And the in about 2 minutes it would start up again. It would'ent run long enough to get the air that was being blown into the house warm before it would shut itself down. Could you help me

Check or dirty air filter.
Check to make sure all the registers are open.
Furnace might be cycling on high limit control.