I have a gas furnace, located in the attic, and very warm air blows from a few vents in the ceiling and coolish air comes from other vents in the ceiling. I do not know what could be causing this problem. Please help.

Judd

It sounds like your furnace is a retrofit in an old house. Hot air registers should not be in your ceiling. Do you know where your return air registers are located? I think it may be time to have a pro come out to your house and see what's going on.

How long have you been in the house? Was it ever any better? Are the vents the cooler air is coming out of further from the furnace? I would go up there and look around. An attic should be as cold as outside. If you have old sheet metal ducts, the air could be cooled off before it gets to the far vents. You may want to insulate them, or even replace them with ones made of fiberglass.

It sounds like your furnace is a retrofit in an old house. Hot air registers should not be in your ceiling. Do you know where your return air registers are located?? I think it may be time to have a pro come out to your house and see whats going on.
Furnace is in the attic! Where else would the supplies be but in the ceiling. Large homes are often built this way so you can better control the temperature on both floors. Also really is great when you need cooling.

My guess is the longer runs take a while to heat up, after all the ductwork is in the attic and they get very cold, that and flex duct doesn't have the best R-factor.