Lately, while cruising at around 60 (2000RPM), my '89 Camry (135K) will for a moment or two downshift, revving the engine to around 3000, then shift back into high. This happens on level stretches or sometimes even on downhills. Any suggestions?

When was the last time the transmission fluid was changed? If recently, then the shop may have put a “generic” fluid in the transmission. To make sure you have the correct fluid you might be best buying it from the Toyota parts dept.

If the fluid has never been changed, get it done, use the correct fluid, and clean out the trans pan and screen. A transmission “flush” is not what I’m talking about. Flushing at this point isn’t a good idea IMO.

Once you have the correct fluid, and fresh fluid in the transmission then see how it works. There are solenoids that control the lock up torque converter and there could mechanic and/or electrical issues with the transmission. Clean fluid is a good place to start, and it is possible that’s all you need to get the tranny working fine.

I had a '91 Camry, and I have to point out that these vehicles were only adequate power-wise even when new. In cruise control on the highway, in one that age, even a gust of wind can change the engine’s need for a bit more power…and cause it to downshift temporarily.

Mine '91 Camry shifted regularly on the highway too. Drove me nuts. But that’s just the way these cars are when they age.

By all means, have the tranny serviced. And get a good tuneup. But if you check the compression I think you’ll find that the chipmunks have simply grown old and tired.

+1 for @UncleTurbo and @thesamemountainbike.

Sounds like the torque converter is going in and out of lock up. Make sure your fluid levels Are correct and then, whether its s solenoid/valve body issue or torque converter, you should either dump the car or look for a used or reman transmission. Unfortunately on an 89, it’s not obd 2, and you can’t read live data to see what the trans is doing. If I were to look at it at my dealership, I’d check fluid level (condition is seldom an issue unless you’ve had a trans cooler leak, and strawberry milk for fluid. If that was good, I’d price a box (trans ass’y). Btw, an 89 used dexron 2 or 3, so generic fluid is a non issue until you get to the early rav4 or 2002 on all the other models that switched to T-IV, and then it could be an issue not on yours. At all.

My first thought is when did you have the last tune up and oil change. It could just be poor performance by standard needing to be replaced or cleaned parts causing a performance loss.