'97 300D Transmission ?

She has 118K; had it for the last 4K in miles.

Symptoms:

Driving along at 70-75 MPH when I notice and slightly feel the engine RPM increase fromt he 2000 RPM range to the 3000RPM range.

Seems to be shifting to the next lower gear. This lasts for 5-10 seconds then returns to the 2000 RPM range as was before.

What may be causing this to occur? Ideas I could present to my wrench for a fix?

TIA

Bob