Engine/Transmission (1998.5 - 2002) trans temp gauge help

[gottorq33]

I have an autometer ultra series anyway I have replaced gauge,wire going to it and sending unit already first time gauge was way off sending unit was like miscalibrated or something now it is only about 25 degrees off at all times on the low side of actual temp and also sometimes I see the gauge like if I go into reverse or something and I would go reel high for a second and bounce around then would calm down fluid is good and it is in bottom of pan what could it be driving me nuts especially with summer and hotter trans temps rollin in wanna protect my baby