Well I had what I believe to be a "qualified, licensed technician"
go over the system and he could not find a problem. The voltage
checked out OK, AMP draw checked out OK, CFM (tested with some sort of
electronic sensor) tested out OK. He was puzzled to why the system
keeps going through motors. He said the bigger (1/2 to ¾ horse)
motor was pulling less AMP's than the systems max, so he doesn't
feel this is a problem. It was not a dirt problem. It was not a
restrictive air problem (at least with the filter).

Question for Roy: Did Mr. Trane test CFM differently and is there a
more through test for the duct sizing issue? And what was the final
diagnosis of your problem?