Help! Furnace blowing cold air. Thanks.

crosely

Hi! I'm new here and really hoping you can halp. I have a Comformaker 90 Plus Furnace that was installed in our Michigan dirt basement in 1998. For the last couple of years during the winter it keeps going out on me. It will work fine for a few minutes or even a few hours but then the hot air that is blowing turns to cold air. If I get up and turn it off for 10-30 mins I can usually get it working again, for atleast another couple of hours then I have to do it all over again. It doesn't happen everyday but it happens very frequently. I have had a repair man out to look at it numerous times and he can't figure it out. We change the filter, he has cleaned it out and it will work again but after an hour or so it will blow cold air again. The pilot remains lit, the blower is on but the warm air turns cold. Any ideas? FYI - due to it being in a dirt crawl space the furnace is installed on its side, hanging from the ceiling (rather then floor above it) and the condensation line ? is installed in a way that it drips out onto the dirt floor. Thanks!