Bad clutch

[njsteve]

No way to really tell by eye. If extreme cases you will see wear/burn marks on only one half of the flywheel. All it takes is to have the center hub portion bent by a couple thousandths and the disc is ruined. If it runs long enough it then wobbles/vibrates and damages the pressure plate. I've seen front bearings go and trans cases crack from the vibration in extreme instances.

In this case you would need to pull the flywheel and resurface it again to make sure it hasn't warped and then replace the disc, pressure plate and throwout bearing.