manual tranny trouble

I have an odd problem with a 91 Cavalier. I replaced a manual tranny clutch last year (new t/o bearing and flywheel truned too) and the car drove perfect for 2-3 months. Then, it had troubles going forward after a coupel of miles. It would always go in reverse just fine. When it cooled down completely, it would go in gear forward, but after a coupel of miles, it would again stop going in gear. The clutch peddle always feels fine. Does anyone have any ideas???

Leaking secondary cup on clutch master cyl when it gets hot, failing to block the return port mebbe? Reverse gear is always easier to shift to, even on the old linkage/cable operated clutches.

Thanks for the advice. If I decide to keep the car, I'll put the tranny back in and check these things. The return port must have been blocked cuz it worked pefectly for a couple of months.