Jim,
Your bike is in good hands with Kim. Give him my regards, I'm the Dutch guy he sold two rims to last year.
As for the 5th gear (and 4, and 3 ...) pitting occurs frequently on the teeth of the gears. Especially on the high geared older models that made such a nice sound when cruising at low revs.
So, open up the engine and gearbox and inspect the teeth of the gears. If any sign of abnormal wear replace!
Picture can be found at
www.xt600.de, look for 'horror pics'.
Few months ago I actually bought a XT600 on which one of the 5th gear gears had burst into 3 pieces while driving, the then owner, doing 100 km/hr during the event, came to a stop with a blocked rear wheel. So it is not a fairy tale ...
Auke