Man, I really hate to sound like a broken record here, but it really sounds like an old fluid issue. Check this
thread here. Follow the instructions I laid out for "bspurlock".
What I think is going on is called "over-run". What happens is that the tranny is grabbing the gear, but it can't hold it. Now! Don't confuse this with slipping. Slipping is when the car just falls flat on it's face and begins to slow down while you are holding the throttle, all the while your engine RPMs race for the sky.
Over-Run is something that an automatic does as a sort of failsafe self protection system. The way it works is that if the tranny for some reason choose's the wrong gear, such as if your Range Sensor goes out, and it is too low the range you are currently in - your tranny will over-run that gear. Basically what it does is allows the clutches to slip a little, combined with slippage of the bands. By spreading out the extra load, induced for whatever reason, it keeps all that torque from ripping the tranny apart.
Just look at like you would a bed of nails. Try laying on a single nail. Ouch!
If you lay on a full bed of nails, you are evenly distributing your weight across all the nails and not just one.
Now I know that I said slippage and over-run are different, but actually they aren't, in principle. Slippage normally happens because of a mechanical or hydraulic failure and is directed to a single element, which means certain death for whatever part is doing the slipping. Spread all that out of the entire tranny, even the gears, and you just saved yourself a ton.
Man I hope I explained that well enough. I know, I tend to ramble a lot with uselessly needed examples and such. I am the only person I know that could turn a simple answer like "Change your tranny fluid!" to a full page write up.
Just do the drain and fill as I described to bspurlock and let us know how it goes.
