Trane Furnace/Thermostat Problem

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
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This is Turtle.
The Honeywell PC-8900 you have may be set up like some that are done and when you see a temperature range away from the set point . You getting into the two averaging of the temperature in the room. one temp is the wall temp and the other is a remote temperature sencer reading and then it will average that temp to give the reading which will give you a fall back temp on the display and it will run the low stage heat to run longer to even out the over all temp in the house. Now when this condition is in effect it should not read lower than 4.2F lower than set point for the second stage heat should come on to bring the temp back up to about 2F range of the thermostat setting. So you could see a temperature on the thermostat display of room temp and it be below the set point by 2F or so and still not kick in the second stage heat because it is a average temp of the house. So this thermostat can let you see this condition sometimes if the right conditions happening at the right time.
Now you can read about this condition at this Honeywell website stated below :
http://content.honeywell.com/yourhome/ptc-thermostats/PC8900fag.htm#showdifferenttemperature
Go to the site above here and scroll down to Question # 32 which will speak about this condition
TURTLE
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