Weird downshifting issue
 

1999 blazer, 4.3, 4x4.
I seem to have a strange downshift at cruise speed.
I use this old blazer as route vehicle on rural mail delivery route and first noticed it while traveling backroads at 35 to 50 mph. Felt like the engine was faltering at steady throttle. Increase throttle slightly and it would smooth out until it caught up to the throttle and then back to just slightly surging. More recently I've noticed it more on primary roads at highway speed. With throttle steady on very slightly rising road where the engine needs to work just a bit, it remains at constant road speed with steady rpm. However, when the grade begins to descend just slightly, engine speed will increase by about 600 rpm while maintaining steady ground speed. Seems to downshift anytime engine load is very light. On level road it cycles back and forth between 2000 to about 2600 rpm at 70 mph. When it downshifts, a very slight increase in throtle will cause it to upshift and rpm will return to about 2000.
No CES light. Other than the shifting, it runs great.
Any suggestions appreciated.

Could be a failing 3-2 downshift solenoid. Usually will give 2nd gear starts and CEL. Inexpensive but messy job. May start there. What condition is fluid?

Attachment 30131

TMan,
Thanks for the comeback.
Fluid is fairly brown, no burnt smell. Vehicle has a little over 340K. Tranny was rebuilt about 3 yrs. & a little over 100K back. No CEL, no 2nd gear starts. Goes 120+ miles/day - start & stop all day for 80 miles of it. I notice that on the first 15 miles or so in the morning on the way in it works normally, then begins the weirdness.
May be just getting to the end of the rebuild.

Failing electronics are very sensitive to heat. Might have the correct resistance when cold but loose it as it warms up. That is a lot of miles!! Good luck.