Why would my thermostat keep resetting?

I have a Clima-touchuch brand digital thermostat for my AC/Heater which i replaced after it stopped working correctly. The digi-thrmostat would continually reset itself whenever i tried to turn the heater/furnace on & it would come on for a few minutes & go off again when the thermostat would reset itself again. Called AC guy said i'd have to order a new thermostat for $475.00. I found one supposed new-hooked on Ebay, got it & connected it up to the receptacle. Whalla...now the new thermosresetset's itself contstantly. Can't access the heater on/off or the AC on/off or the settings on the digithmostat before it reset's itself again....and again. Is it likely the thermostat is faulty as was the older one i tried to replace? Should I try one more time with another (new) digi-thermostat? Could this be caused from a faulty transformer on the Air Hanlder? I tried to reach the Climatouch Company & AED electrons in Canada-they are out to lunch. Can you help? Thank you!