Hello,
It seems if I cross the red/white wires my furnace will run all day but when I hook up a thermostat (programabel, non-prog, mercury.. etc etc, I have tried 7). It just won't keep the house warmer than 60 deg. It will turn on and off repeatedly and with some themostats just not work all together.

The strange part it, after installing a new thermostat everything seems to work fine for a few hours.

Any thoughts? Thanks.

That is strange. All any thermostat does is connect the red and white wire when it calls for heat. I could see one bad thermostat, but 7 fail in the same way? Have you checked your voltage? Should be 24 volts. If it was wrong, I would think some of the furnace components would fail too.

I agree.. it's making very little sense. The only more information I can give is, originally my programmable thermostat worked great but recently I had a 'green' wire installed so I could run the fan in the summer. It has be temperamental since then.