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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.



Shifter linkage is good, I have checked it more than once for this problem. As far as the z-bar flexing goes, I guess the only way to solve that is a torque strap since there's really no way to see if its flexing when i shift while driving.

As far as the fluid goes, im starting to think this may be what it is. I have been told that synchromesh does have synthetic additives in it. Finding GL-4 gear oil here is like finding a needle in a hay stack...was thinking of trying ATF since it seems quite a few people use it with no problems, but im not sure what type of ATF would be suitable? Dexron? Mercon? Type F? multi purpose?

I still have the drained synchromesh fluid that I can put back in, but i thought id try something else first to see if it helps the problem go away