Transmission shifts hard from first

I have a 1999 SE with 3.4L V6 and automatic tranny. I've noticed that it will often shift very hard from 1st to 2nd, especially at high RPMs like merging onto a highway. Any time I've had to do work on the car that requires disconnecting the battery, I notice that the problem goes away for a week or two and I get very smooth 1st to 2nd shifts. Then the problem returns. I've changed the fluid and filter on the recommended schedule, but have not flushed and don't intend to because of the problems that usually causes, but with the way the problem comes and goes when the battery is disconnected, it seems like this might be more of a computer or electrical problem than a mechanical problem. When I hooked up an OBDII scanner, I didn't get any codes, though. Has anyone else experienced this?

I am by no means an expert on this subject, but I do know that on some of the newer cars, the vehicle's computer has to "relearn" certain driving characteristics after the battery has been disconnected for some duration. It does this relearning by monitoring the users driving habits for a certain period of time. If you drive timid during this time and then "get-on" it afterwards it will appear to effect the driveability.

The car is basically driven under the same circumstances every day--it's a commuter car and I take the same route to and from work, which amounts to about 65 miles round trip. I actually had the same thought as you, so the last time I disconnected the battery, I tried "pushing" it a bit and accelarating harder than I normally would. The tranny was smooth as can be. Two weeks later, back to the hard shifts.