I just moved into a 1991 mobile home with a heater of the same age.
The furnace is a Nordyne Intertherm model MGHA 056ABFC-01
Forced air and natural gas heater.

The problem is with the fan
I turn the thermostat up to turn the furnace on.
Once the burner heats up the fan turns on,
the fan only stays on for 30 seconds then the fans turns off for 20 seconds,
then the fan turns on for 45 seconds and then the fans turns off for 20 seconds,
then the fan turns on for 45 seconds and then the fans turns off for 20 seconds,
then the fan turns on for 50 seconds and then the fans turns off for 20 seconds,
then the fan turns on for 60 seconds and then the fans turns off for 30 seconds,
then the fan turns on for 70 seconds and then the fans turns off for 35 seconds,
then the fan turns on for 75 seconds and then the fans turns off for 35 seconds,

and so on until the fan will run for about 90 seconds before it turns off.

I had a technician out to look at it and all he said he could do was replace the thermostat switch in the heater. He did that and the problem still exist.

Any ideas what is wrong with it or is that how this model operates?
It is very inefficient at heating.

Rhonda

A fan limit switch is what brings the blower on in heat. the heat exchanger(or fire box) has to be at a certain temp to bring the blower on(around 120 deg). once the blower is running the cool air moving across the blower control can shut it down until the heat exchanger heats back up. In short you may have a weak blower control.