I have a Trane XL80 two-stage with a Honeywell PC8900 programable
thermostat. Both were installed about 30 months ago, and everything
was functioning normally.

Twice in the last three weeks the temperature dropped a few degrees
below the set-point during the night, even though the furnace continued
to fire and blow at the lower stage and fan speed.

The first time it happened, I turned the system off at the thermostat
and turned it back on and the furnace turned and and the second stage
kicked in and the blower came on high. HVAC repairman came out and
could find nothing wrong with the furnace, but he didn't check the
thermostat.

This morning the same thing happened, but I didn't reset the
thermostat. After a time, though, it did "kick-up" itself and is now
operating normally, again.
Any ideas would be greatly appreciated!

Thanks.

Bill