Well, it seems to have been a thermostat problem, as my installer thought it was.
With his help, I installed a new thermostat.
Installed a Honeywell Focus Pro 6000 (TH6110D1005).
Since installing the thermostat, the furnace has run continuously until it reaches the set temperature.
Most times it has been from about 62 degrees to 68 degrees......runs non stop, no problems.
But I finally went a away for a weekend and gave the house a chance to cool down to 55 degrees.
Set the temp to 68 degrees when I got home and the furnace ran non-stop until it reached 68 degrees. Took the furnace about 45 minutes or so.
Problem solved. Excellent.
Thanks to everyone who helped.