Engine/Transmission (1998.5 - 2002) 01 transmission shifitng

[BGoodale]

after a service last year and an install of a transmission temp gauge , the transmission shfts down from 3rd. or overdrive out to 2nd. at 35 mph where before it was more in the 20 t0 25mph range. Everthing else about the function seems to be normal. If I keep the selector in drive with the overdrive in then the downshift occurs normally until I have run the truck for awhile and all is warmed up . Then it shifts fine sometimes and other times shifts too early. Do I have a serious problem or a simple computer problem?