97 JX downshift for no reason

I have a 97 JX convertible with a shifting issue. The trans was rebuilt 60K ago and has worked flawlessly until now. It is downshifting unexpectedly at highway speeds. It will then shift back into overdrive. I can go thru all the gears at strong throttle with no problems. It is also sometimes downshifting hard when coming to a stop. Could this be solenoid related? The fluid is full and looks clean and red on the dipstick.