Transmission problem? '01 Xterra

I have what appears to be a transmission problem.
- after engine temp is normal (initial start up of car in the morning or afternoon)At 60 mph,
I read 2500 rpm on tach. After 5-10 minutes of steady 55-60 mph, the rpm drops 500 rpm to 2000
(where it should be)then all is ok.
This is not a shifting out of O.D. problem. As I kick it down to 3rd gear, it will sometimes clear the problem. You can sometimes feel the car move into what feels like a slightly higher gear when it settles to 2000 rpm. Odd behavior for a xmission.
This is eating my gas up bad. It has gotten worse in the past month. Dealer "can't replicate" the problem (conveniently) yet. No engine codes are evident.....
Any clues?
32K miles, no other problems, just had 30K checkup and replaced xmission fluid just to see.

You're right, it's not an overdrive problem. Engine temp is irrelevant. It's the torque converter not locking up until the temp of the transmission fluid is up to normal. When it does lock up, it will shave that 500 RPMs off. It takes a few minutes of driving to bring it up to temp.

It's normal,
Brent

What's really frightening is that the dealer couldn't replicate the problem, when infact there is nothing to test for. Indeed this is normal behavior for automatic transmissions. I might also add that I believe it's covered in the owner's manual AND has been covered here many, many, many times.