I just replaced the motor in mine - I posted this in another thread:
Your best bet is to look at any internal friction between the spring coils. Take the spring off and clean and lube between the coild. Get any rust out from between the coils. The rust really binds the spings when lowering the windows - that is why sometimes they go down very slow and get stuck but come up OK. Get some grease inbetween the coils then reassemble. The regulators really need to be cleaned and freed up. Clean out all the old gunky grease between the gears and hinges, flush out the rollers with WD40, then add motor oil to keep them moving free. Take the springs out and the motor off and cycle the hinges by hand to find any sticky areas. It all adds up to help the windows roll up and down better.