1990,350 SDL with sticky upshift problem
 

Hi I've got a 1990 350SDL with @198,000 miles on her, she runs great EXCEPT if on highway runs of over an hour recently she has begun to labor to get into 4th gear and will after continued driving will also develop problems going up into 3rd. After the vehicle is stopped and shut down for 20 to 30 minutes it will be fine running upto highway speeds again for about and hour...We have been pulling our hair out and had NO luck tracking down this gremlin!! Anyone out there have a similar problem?? She runs too well for it not to be an overlooked item!! Thanks!

Try replacing the filter.

As the reverse clutches go they can fill a filter in just a few miles. You problem is classic symptom of a packed filter.

If that don't work I would loosen and retorque the valve body. Internal leakage can cause such temp dependant problems.