how old is the clutch? have you every had the tranny off? if so did you check/replace the pilot bushing? i have had this same problem goin on for a bit now and just replaced the clutch and flywheel. in the process i replaced the pilot bushing, but get this, there was NO pilot bushing in there in the first place. there was the pilot converter which is used for the auto tranny. so i came to the conclusion that my motor was shipped from japan as an auto and someone swaped the 5spd on to it to sell it for more money. pretty shitty if you ask me. the pilot converter will not allow the clutch to fully disengage and it only gets worse as it gets warmer. so drop your tranny, check that you dont have a pilot converter instead of a pilot bushing, and be ready to buy a new disk and pressure plate and possibly flywheel cause chances are your old ones are fucked. hope this helps you out. if you need a picture of what the pilot converter looks like here it is
http://www.zilvia.net/f/showthread.php?t=90797
hope this helps you out.
mike