legacycentral bbs

I have a 92 legacySS. It has 101,000 miles on it. Here is my issue.
When I take off from wherever when the transmission is cold, the car will start to accelerate just fine. At about 40-50 mph when it is about to upshift the rpms come up and the car will accelerate no more. Just like 4th gear is going out. But after a few miles of high rpms once the tranny is warm it will shift into 4th, and then I can resume normal driving.
Also when I take my foot off the gas at highway speed it will seem like it wants to down shift but doesnt. When I am at a standing start if I hit the gas too hard the car will lurch forward. Again the car will resume driving normal right after this happens.
When I bought the car the previous owner said that the fluid and filter just needs to be chaged. I went and had that done almost 5,000 miles ago with no improvement. I even added a trans. additive to the fluid.
I have two questions regarding this. One, is it possible that it could be a part in the transmission that is acting up and not the transmission actually going? I have read that there is a one way valve for the torque converter that is supposed to keep pressure constant inside that can get fouled and needs replacement. My second question is if the whole transmission does need to be replaced, what is the cost difference to turn it itno a 5mt?
This is my daily driver right now and I am in desperate need to get it fixed right away. I do not want to broken down on the side of I-80 in December. Thanks for all your help.

The transmission will stay in 3rd gear until it warms up if the ambient temperatures are very cold. That may be what you're experiencing.

The check valve is internal to the trans....it may be the problem, or there may be something else wrong internally. You can check out my swap write-up and my parts lists where I've got costs. Not all are for the 5mt swap. Depending if you can get a donor car, the price will vary, especially if you do the work yourself or take it to a shop.