88 Camry; Rough Gear Change

[kohokoho]

I have recently noticed a rough transmission change at around 40-60mph in my 88 Camry (automatic transmission) with 163K. As some fellow users suggested in other threads, I tried easing up on gas when the transmission is rough, but doesn't help much. The ride becomes smooth, of course, after the car picks up speed again. Other than this the car runs incredibly smooth.

Any ideas what the problem might be? Would it be that it's time to change the transmission oil/filter? I don't know the last time it was changed, i got this car when it had 158 K. I have, however, changed the engine oil every 3000 miles, if this helps.