I have a 2005 3.8 NA with 80k miles. Recently began shifting roughly from 1st to 2nd, 2nd to 3rd etc. It is sporadic and only seems to happen when the car has been driven at least 30 minutes. Also seems to happen when the weather is warmer (70-75F).
I changed filter and fluid. No help with the problem. Don't seem to have any codes.
Consider adding seafoam transfix, drive 50 miles, then do another fluid change and this time to Dextron VI, also add a shift kit while doing the fluid change. Had your exact problem 30k miles ago and the above fixed it for me. Trans is running better know than it did after the fluid change- it is actually getting stronger with time/ miles.
Had the same problem with mine, probably a malfunctioning PCS. Installed a shift kit a couple of months ago and the problem went away. Check with BillBoost or Danthurs to see if they have any shift kits for sale.