Bizarre transmission issue-Help please!

I don't know the answer but I know where I would start looking:
Silentbloc bushings - one right in front of the engine and one right behind the shifter
Motor mounts and check for clearance between the shift rod and exhaust, etc., look for evidence of rubbing
Clean out and lube the shifter box (under the gate in the center console)
Shift shaft seal issue - even though it is not leaking does it swell or bind, maybe only when really hot?

 

You know, that a good question. I think perhaps different parts fo the car might see different amounts of heat depending on the circumstances. Perhaps at freeway speeds the engine gets more air cooling but the radiator hoses are running hotter? I don't know. Maybe the tranny runs hotter on the back roads from hot cats and less airflow?

In any case I wouls look at all the external factors before looking inside the box both because it is easier and because most transmissiion problems around here (thankfully!) seem to be outside the box.

 

My guess would also be that it's related to shifter clearance/binding. Probably the reason it's most apparent on a long steady run rather than a mountain/city romp is that the mountain/city travel likely allows the binding to periodically clear itself with the relative movement of the linkage v. all the potential areas of binding/interference.

As suggested above look at all of the shifter mechanism - bushings, potential points of contact with engine/chassis, lubricate all joints, etc. I suspect you'll find it with careful examination and cleaning. Good luck!