I had the same problem.
The car would shift fine, except after 6k rpm.
Then gradually, it would get worse... and eventually got to the point where I couldn't shift past 3k rpm.
Took a look at the clutch and it was messed up in all sorts of ugliness. For one, the modular assembly was not longer secure. Second, there was debris all in the bellhousing.
Might wanna take a look at your clutch.
It does have a lot of miles on it.
Mine went out at 70k, but that was from me being a dumb @$$ and launching like an idiot all night at the track.