I have experienced the same symptoms on my '01 300EXC. I back bled the system, examined the slave cylinder and clutch, found nothing usual. Over on the HS BBS someone suggested that the clutch fluid was over-heating and that I should try ATF. I guess Lafferty does this, so anyway I changed over to tranny juice and my problem did not reappear during the last ride. It was 15-20 degrees colder though, so not really conclusive evidence. My clutch did the same as yours, went flat after accelerating hard out of a corner or after an obstacle.
So does this problem only occur after the bike is fully warmed and you are riding aggressively, and do you a slave cylinder guard installed?
Previous thread here was :
"http://www.dirtrider.net/forums3/showthread.php?s=&threadid=27507"