Transmission Overdrive Thunk

[PDPerry]

It's been this way since the vehicle was new. Only really notice when on long road trip.
Like I'm on right now; Sacramento to Winter Park, CO
Scenario: Driving at highway speed. Cruise control set to 70 - 75 MPH. RPMs between 2400 - 2700.

• Start going up a long incline.
• First she drops out of the torque converter lock with commensurate slight increase in RPMs.
• Shortly thereafter, she kicks-down into 3rd gear. RPMs increase to 3700-3900 RPMs. All good.
• Crest the incline, she stays in 3rd for a bit, then BANGS back into 4th OD.

Anyone else? Any thoughts on cause? Something sticky in a transmission valve?

1998 LC. Member of 300,000+ mile club. Well maintained (just did steering rack & valve cover gaskets before this trip!)

 

[Gartenmeister]

My 2000 with 335k did this. I replaced the fluid (which looked fine but was of unknown age) and that shift is now much, much smoother.