Sounds like a combination of issues,
The grinding/vibration at an idle is probably the throwout bearing, or possibly the front bearing, both can cause that.
The popping out of third can be synchronizes, shifter linkage adjustment, bad bearing, or bad spot on a gear.
The front bearing is probably the most over looked part of an 833, they get almost as much abuse as the throwout bearing, and go out pretty frequent, it should be checked anytime you change the clutch, a bad pressure plate that has any loose parts, or a clutch disc that has uneven ware can cause it to go out very fast.
