Bizarre transmission issue-Help please!

I have a most bizarre transmission issue. If I drive my car more than 80 miles on the highway in one shot, the shifter gets very notchy and begins to feel like I am rowing through molasses. This problem does NOT occur if I drive the same distance, or even double on backroads or in the city (shifting regularly). It really only happens if the car cruises for very long distances on the highway. I have changed the transmission fluid-no luck (now using Redline NS), master/slave clutch cylinders are fine, no air in the lines, I have no problems with the clutch (its not slipping and Algar says it takes up within design specs), the transmission shifts perfectly according to Algar and my personal mechanic, and not one seal is leaking anywhere. It is the most bizarre anomaly and, quite frankly, it has everyone stumped. I am not particularly concerned only because I so rarely take the car on the highway for long trips (maybe 2x since I got her), but nonetheless, the problem solver in me would like to get this sorted out.


Any suggestions?

I want to get a whole bunch because I have to devote a day to driving up and down the highway before I can even reproduce the problem to a significant enough degree to start running diagnostics.

 

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?

 

Thanks for your suggestions, its worth a look. As pertains to the shift shaft seal, Do you think the car would get hotter on a droning highway drive or a spirited mountain romp of equal distance (not really sure myself).

 

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!