Shift Problems

Make sure trunion is tight (under driver's side header) and that its bracket's welds to frame are still fully secure. If so, you might then also check the 2 allens on underside of shifter shaft that enters the little shifter box on driver's side of the ZF, accessed by pulling the 2 rubber plugs from underneath that box. All are easy checks and possible sources of slop. But you may still have a slight misalignment of the linkage needing adjustment. I think when things heat up and things get in motion & under load, all that can change slightly.

Sounds like a problem I had once which ended up being a simple adjustment at the slave. Check your clutch freeplay and adjust accordingly.

If it turns out not to be the clutch, another source of slop in the shift linkage are the bronze bushings (and possibly the steel shaft) in the shift lever. See:
http://www.panterasbywilkinson.../catalog/ill10a.html
Parts 18 and 19.

Wilkinson has (or had ~12 years ago) a kit for this. I had that problem at about 100k miles after replacing the trunion and shaft with one that had slightly more slop than my original. The obvious symptom of excess slop was that 1st and rev were fine with the gate out but not with the gate in. If I adjusted to get them to work, then 5th got messed up.

As I remember, the indicator that the shifter bushings were part of the problem was the free play when the shift lever was in the centered neutral position. Using almost no pressure, the shift lever moved from side to side maybe 1/4 to 3/8" before any resistance was felt. After installing the kit, there was almost no free play.

Another source of slop in the linkage, of course, are the U joints. I had replaced all of them with new ones, so that wasn't part of my problem.

@ Buttondoor, good tip on the allens in the shifter box.