1992 Toyota Camry Perfect Advice!

Transmission problem
1992 Toyota Camry 4 cyl Two Wheel Drive Automatic 359k miles

Hello, I would like to get an advice regarding my 4 speed auto tranny (3sf), it is not yet fully equipped w/electronic stuff. There are times that it changes gear slowly and there are times that it changes so fast and smooth. Or sometimes downshift itself, means while accelerating-it doesn't gather speed. It will take about 10 sec to recover then speeds-up. I had changed my ATF a month ago, but never changed the tranny filter a year from now. I would like to change the ATF again w/ flushing, and also change the filter. If symptoms persist, should I need to change the auto tranny? Hoping to hear from you soon. Thanks