Thermostat issues

i have a suburban sf-30 furnace that quit working. I replaced the mother board because i found a burnt part on the old one.I started the unit by crossing the blue wires (for the thermostat) and it worked fine. hooked up the thermostat(manual- not digital) and it did not make a sound. I turned it on a/c and it just hummed. i replaced the thermo, the furnace and a/c worked fine. after about 6-8 hours of the furnace running at a set 68ish degrees it stopped. the new thermo did the same thing as the old one, no furnace action and a/c humming. i went back to the blue wires on furnace and cross them and furnace worked fine . i thought i must have gotten to cheap of a thermostat, so i installed another one and it worked for the same 6-8hrs then nothing. I've got ground hog day going on.any ideas why my thermos are failing? they all had power to the thermostat as follows: 7.5v to the 7.5 post 8.1v to the cool post and 12.8v to the furnace post as well as all the others except ground.The furnace unit still works by crossing the blue thermo wires at the furnace unit!! new member/first time poster