When my Daewoo Nubira gets warm the gearbox changes become very jerky. The problem is worse at medium speed when the gear changes down under acceleration and then changes back up again.

Problem only occurs after 30 + minutes of running.

Try using Amsoil Synthetic Universal ATF or Mobil 1 Synthetic Multi-Vehicle ATF. You will be amazed how much better ATXs shift with synthetics. These are thoroughly "battle-tested" products, that I have used for years.

Drop and clean the pan, change the filter, and replace with synthetic ATF. Try and drain the converter, while you are doing this. Otherwise, you only change about half of the fluid. Check Amsoil's website on how to change all of the fluid. This is very important on Daewoo products, because it's difficult and expensive to get transmission parts for them.