Studder in tranny in overdrive

Good Afternoon,

1999 -early body style, z71, 5.7, auto tranny.

When going up hill at 60-65 mph (even when empty) the truck starts a little studder in the tranny. I can move the select lever into third, it kicks in and runs smooth as can be up the incline and I shift back to D and all is well.

If I feather the throtle I can make it up the hills ok but the speed slowly reduces due to the throttle control. The transmission works perfect except for this quirk.

Not that it matters - but the drive system was serviced by a GM dealer prior to my purchase with all fluids replaced. The previous owner had all work done at GM with many, many PM's. It was/is well taken care of.

This problem didn't come with the vehicle but has slowly crept up as time has gone by over the last 12-14 months.

Any helpful suggestions is appreciated.

Merry Christmas !

Always make sure your engine is running up to par first thing.
If engine is not running right the transmission will not work right.
How is the tune up end?
How many miles on last tune up?

Tune up in the last 20k with plugs, cap and rotor. Starts right up, no miss or oil usage. Purrs like kitten at start up. 195k miles. Regular maint at 3k intervals. Highway driven it's whole life with no heavy loads. No coolant leaks, oil, tranny, or rear end leakage. No cross conatmination of fluids as well. Truck appears perfect in looks, ride, start up, in town and highway driving all all the rest. ......... just this darn studder as described.

Thanks again.

If a staedy studder/miss jerking after od lockup and under a load it is usually engine running problem.
If just going into or out of lockup it could be transmission problem.

Get it on a good engine transmission capable acanner and read out all engine running and transmission codes and data.
Also run a full fuel pressure test.
Post back any codes.
Let us know what you find.