Clutch fade. You've heard of brake fade? It's the same thing but wtih the surface of the clutch and flywheel heating up and not grabbing properly.
P.S. If it happens in normal driving it can sometimes be an indication of the clutch wearing down too thin, but since you were "racing" I doubt that's the case.
Kevin (President TBEA)
"WAAAAAAARRRRRIORS, come out to plaaaaay."