Woth checking the selector rods!
You might get lucky just changing some of the worn out plastic joints in the selectors unit
They are cheap plus a weighted rod might help make selection smoother if you dont have one in there already?
+1
Worn linkage can make gear changes terrible.
I made new bushes out of Bronze and bought a kit from TSR (i think) which replaced the plastic UJ's with metal ones.
It is one of the best upgrades you can do to a Mk2

If that or the oil change does not help or cure your problem then its a box rebuild
