Air Cond. and Heater Problem

My air conditioning and heater will stop working for a few minutes, hours, or weeks ; then start up suddenly. The fan doesn't blow in any switch position and i don't hear the vacuum operated ducts operating when the problem is occuring. I replaced the relay and checked the pressure switch; it's ok. HELP ME ! PS When air is working it's great--VERY COLD and heater is fine when problem is not happening.

So you don't think you have a bad blower motor? Sounds like it to me.