Engine/Transmission (1998.5 - 2002) Auto 2nd to 3rd shifting problem

[BBoyle]

Stock 1999 2500 2WD, with 244K, showing no codes. Cold trans seems to shift normal. As it warms up, I have to let off throttle to make it shift from 2nd to 3rd, and sometimes 3rd to 4th. OD & lockup work ok. Down shifts OK. Take off near WOT it shifts thru the gears OK. Possible TPPS problem? Any ideas or suggestions greatly appreciated.