[Zeke]

I don't think you are going to know what happened until you monitor the T-stat and see that circumstance again. Why not buy simple digital heat/cool T-stat with no timer settings? That old analog mercury unit has got to be on it's last legs.

Hope it's not an internal problem, that could be more expensive and somewhat dangerous. You might also try to find a small independent service guy and have you high limit switch checked out amongst other things.