I assume you have looked closely at the brass middle of the bushes as these do wear and go ovoid.

They should fit snugly on the shaft of the linkage of the gearbox.

The bearing upgrade is well worth doing here.

If as you say the bushes in the cables are good then next thing to look at is the brass cube under one of the levers of the linkage on the gearbox.

Again these can wear significantly causing slop in the linkage and resulting in an imprecise shift.

The fix is to rotate the bush 90 deg to run on the other 2 sides.

Sorry I don't have pics but i'm sure a quick google search will bring up some.
