Re: I`m cold...
I have to disagree. Last fall the same sorta thing happened on my truck. The motor would not spin on low, and would be lagging on high. I replaced the motor and viola, it was fixed. If the resistor is bad, I think it will work on some speeds, and not on others because each speed setting is a seperate coil and one bad coil won't effect them all. I got my motor for less than 40 bucks at a local parts dealer. I had to trade in my old one, and keep the fan blade to put on the new one. Pull out your old fan and see how easily it spins, if it doesn't turn freely, it is shot. Also check to see if the ground wire that plugs into the motor is corroded, that gave my friends trans am the same symptoms you describe.
There's somethin' women like about a pickup man