Just a suggestion, you said you are using the GM syncromesh fluid, maybe for your application try the GL-4 Rated 80-90wt gear oil, or the Passon performance fluid. It's possible when things heat up the syncro is not grabbing the cone enough maybe the fluid is too thin or incorrect. Also check your shifter linkage throw to see if reverse is being fully released to the detent, so it can't slide over and lock 2nd gear up.
Another thing is if you are making some good power and your engine mounts are bad the engine might be torqing too much and the z bar moves, not letting the clutch release enough.