Auto transmission issues come in two flavors, A) slipping/shuddering usually caused by worn clutches/bands and B) slow/erratic shifts usually caused by pressure issues. Type 'B' will eventually lead to type 'A' issues because the clutches and bands don't engage quick enough and slip, creating accelerated wear. It doesn't sound like that is your issue, yet. So let's look at type 'B' issues.
First step is to ensure your Throttle Valve (TV) cable is adjusted properly. The cables eventually stretch, especially at that mileage. This cable runs from the transmission to the throttle body and simply put, regulates the transmission fluid pressure based on throttle position. It also controls the kick-down when you stomp on the gas to accelerate quickly. Several videos on youtube for the adjustment, super easy and quick - you won't even get dirty!
If that doesn't fix it, the issue typically lies in the valve body in the transmission. This is the brains that controls the movement of fluid for gear changes. When you hear people talk about their transmissions failing after getting flushed, what has happened is sludge has found its way into the little channels and prevented the check balls from sealing or the pistons to move freely. When this happens, the clutches/bands do not engage properly, slip and burn up. Obviously that results in a blown transmission requiring a rebuild.
The value body is fairly easy to remove and can be cleaned without pulling the transmission. If you are reasonably handy you could drop the transmission pan (draining all fluid), remove the filter (2 screws) and then remove the valve body (8 bolts, if I remember correctly). There are some electrical connections too and you have to remove the shift and TV cables from the selector shaft. The service manual explains how to remove, disassemble and reassemble everything and there are also youtube videos that walk through the process, although they have the tranny out. There is only really one pain-in-the-butt part and that is the small gear selector ball- have a buddy around because you need the extra hands to reassemble it. Take lots of pictures so you remember where everything goes, although the parts really only fit in one place, except the small check balls - take pics so you know where to put them. Clean everything completely with lint free wipes - dust/cloth fibers are your enemy.
Reassemble valve body into transmission and refill with ATF. Re-adjust TV cable and run it. It will shift better and assuming the clutches/bands are still reasonably good, will buy you some more time before having to pull it for a complete rebuild.
