This has been answered so many times. Here's something to get you started.

I have been thinking of doing the same thing with my 89 SR5 V6. It has 140k on it and i have no idea if it has ever been changed. It has some weird issues with shifting that make me nervous about tinkering with the tranny, doesnt shift down into 1st when I stop on a fairly regular basis. I have to manually shift it and it seems fine no slipping. So it could be electronic or possibly even vacuum? Anyone have any thoughts I suppose changing the fluid could never hurt but a power flush makes me nervous.

Gadget addressed a problem in that link I posted above that sounds just like that. It was the filter gasket leaking air into the pump. All of you guys with high miles and unknown history, the first thing I'd do is replace the filter and clean the magnets in the bottom of the pan. Then after a refill with new ATF consider a flush next.

I am scared to flush my tranny because I am at 200k mi on my truck and I bet the fluid wasn't changed very often. So I started draining and filling at every oil change and plan on doing this a total of ~5 times depending on how the fluid looks. This will get most of the old fluid out of the transmission don't you think?

I dont plan on flushing it, just changing the fluids. I dont even no how flushing came about in the thread.

I have had 2 auto trans with problems going into gear and gears going soft after 10 minutes of run time. I put in a new filter and my 4.8 qts of AFT and good as new. I am amazed, I was under the impression changing fluids was asking for trouble with an Auto trans also...however it has been pointed out to me that Toyota is real fussy about having clean fluids at the proper fill levels. Just my 2 cents.