In your sig you say you have a trans temp gauge. Where is that gauge's sender at? Is stock sender still in hot line coming out of trans? Sounds like either stock sender has gone bad or if sender has been removed, the stock wiring harness going to sender has rubbed on something and is shorting out. If stock sender is still there, remove plug, solder in a 1000 ohm resistor, tape up plug well and tuck it and zip tie somewhere safe.