Matt,
I think that you are on the right track. Is the master cylinder known good? Flush and fill fluid using high quality fluid.... Make sure the the release bearing is doing its thing and releasing completely. Then......and perhaps this should be first....Make sure there are no fluids finding their way to the clutch assembly.....no motor oil from a leaking rear seal or O-Ring.....and no transmission fluid from a leaking front seal of the transmission. It takes very little of either to make it slip and it doesn't have to be leaking onto the ground under the car for it to be leaking badly enough to get to the clutch. If the clutch disc is at all oily, or has ever been oily without having been really well cleaned, slippage will likely occur at some point in a session.
I just went through the very scenario that you describe. My clutch would feel and operate perfectly for a 30 minute SARRC race and then fade mostly completely away and be significantly useless for the last some-odd laps of a 45 minute Carolina Cup series race; fixed the leak(s) and problem solved (well, I put in a new clutch, too, just to be safe).
Another thought.....are you running a race clutch or a stock clutch? If stock, and assuming you have verified all else is good, then you would do well to consider a different clutch assembly.
Check the potential leakage thing out very closely....you can see the front side of the flywheel, and a little bit of the bottom of the transmission bell housing by removing the small metal cover at the top of the rear housing of the motor and looking in with a flashlight.
Hope this helps.
Cheers.
Gary Gentry
NC Region
#4 IT7/ITA
2008 CCPS ITA Champion
2008 SAARC IT-7 3rd Overall
2009 CCPS IT-7 Champion
2010 CCPS IT-7 Champion
#4 Car>>>Sad State of Affairs
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