I have a Goodman heat pump that's 5 years old (the original heat pump installed when the home was built). Today I turned the heat up from 65 to 70, the heat pump started running (it should have since it was less than 70 degrees in the house). Problem is, it got up to 75 degrees before I noticed it was getting hot in the house. I turned the heat back down to 60 and waited for it to turn off. 10 minutes later, it was still running. I turned the system to "Off" and waited for it to shut off. Still didn't stop running and it was now almost 80 in the house. To get it Off, I resorted to turning the electricity off at the breaker box. I waited hours before shutting the power back on. The system was still set to Off, the fan on auto and the temp set to 60 (still 70+ degrees in the house). As soon as I turned the power back on, it started running immediately.

This thing is possessed! What's wrong with it ? I'm hoping I just need a new thermostat but am afraid it's something more expensive.

HELP!