Manual Transmission - has anyone put a stock c5 clutch kit in a gto?

i put one in mine and 6k miles later first gear started grinding on me at the idle. but if i clear out the gears it slides right in no problem. im hoping its not the synchros. (sp?) has anyone come across this problem? im thinking of putting full synthetic atf in to see if it helps any. someone please help me out. :bang: