Intertherm model MGHA-090-AAPQ -- combustion air keeps running

Our intertherm gas furnace works well except for the fact that the combustion air fan will not shut off untill we kill the power. It has run for 6 hours. The furnace will cycle properly but the combustion air fan does not shut down after a heating cycle. When we kill the power and let the temperature drop enough to call for heat, we turn on the power --- the combustion air fan comes back on and the furnace will go through a heating cycle and at the end of this cycle the furnace will shut down this time the combustion air fan will also shut down. This problem is intermittent. It has gone weeks before it happens again.
Where do I look to fix this problem?