Came home Sunday and heat wasn't working. Checked all breakers and disconnects. All good so I opened the unit in the attic and found a 5 amp fuse blown. Replaced fuse and unit ran for about 1.5 hrs then failed. Blown fuse! Replaced again...ran for ~1.5 hrs and failed. Called a friend and he suggested: 1) check 24v control wiring for nicks that may be causing short circuits. Did that at the indoor and outdoor unit and thermostat. Found a few that were questionable so clipped, stripped and re-connected. Replaced fuse...ran for ~1.5 hrs and failed. 2) replace thermostat. Replaced my old analog mercury switch thermostat w/ digital and replaced fuse. It ran about 1 hour and cycled off. Came on and ran about 1 hour and failed. So what do I check now???