I have a bryant plus 90 that was installed in 1989 Model# 398AAW060120D and it works great most of the time. In the morning is usually when this happens. After the furnace has gone through a normal cycle the blower motor ONLY comes on for about 1 minute then shuts off for about 1 minute and repeats this cycle until I shut off the power to the furnace. I have had 3 techs out and have replaced the main board. ANY help? Mike

There is a thermostat that controls the blower. It could be in the form of a thermocouple, snap disk or some other heat detecting device. Some hot air units will run the fans on and off until the plenum(heat chamber) has cooled off. Two reasons- extract as much heat as possible and cool the until off to a safe temperature.
I would have the tech come back and check anything that connects to the main board... that does not sound like it is the problem.