My 13 yr old furnace fan blows 80% of the time when I use my A/C . Occasionally, the pipe from the unit outside leading inside (I don't know this stuff at all)gets frost on it, and the fan won't blow. I then turn the A/C off, wait until the frost thaws fron the line and start it again. Almost always the fan will work immediately. Now that it's getting cold, I tried the heat and everything seems to work fine, but the fan won't turn. Any help is greatly appreciated.-Mike