Odd Transmission Behavior?

I have a '99 GA SE, 3.4L V6 with 4-speed automatic transmission. For years, it has often shifted hard from 1st to 2nd, especially at high RPMs. No mechanical transmission problem has ever been found, and if the battery is disconnected for several minutes or more when I am working on the car, I have found that the transmission shifts smoothly for the next 100 or so miles before reverting back to its usual behavior. Several people have concluded that it's an electronics/computer issue, but no one has been able to diagnose precisely what is going on. Now, when I am in 4th gear at highway speeds and need to give the car a little more gas, the transmission will often "jump" as if the car's computer can't decide whether it should stay in 4th or go back to 3rd to generate the extra torque. If I let up on the accelerator, the problem stops. Also, if I give it more gas to clearly force it to 3rd gear, it shifts smoothly back to 3rd. It's only when I give it a little bit of gas to maintain speed that it happens. I get no codes off of the scanner, and as I said before, nothing mechanical can be found. Any ideas about where to go from here? Would just living with the problem eventually lead to transmission damage or failure?

How many miles are on the car?

It definetly sounds like a computer malfunction. I did experince something similiar to what you are describing, what the dealer told me that there was a bulletin about if this occurs that they will reflash your pcm to correct the problem... plus they jiggled some wires ( although i already knew that none of this would help my car) but maybe this is something that needs to be done for yours. The problem could potentially lead to early failures, it's better to be safe then sorry. Good luck

128,000 miles.

128,000 miles

Probly time to try changing the fluid and filter, this might clear it up a little.

They do shift hard from 1st-2nd.
Dont know why.

Btw, if you are scanning it with a typical scanner, they all dont pick up trans codes thrown by the pcm. A dealershit or transmission shop can read/check those for you.

Thanks. I changed the fluid and filter at 115,000, and I'm not getting any OBD II codes, so I think it's off to the transmission shop. I try my hardest to avoid the local Pontiac dealers here.