Engine Temp. Gauge Issues

In that case I don't know. Are the senders you changed the same/ similar manufacture? Maybe they are simply "wrong." Maybe attempt to find another independent sender source

Again, you could check resistance.......When the gauge is heading towards the "overly high" reading, pull the wire off, check sender resistance, and if it indeed is lower than the 10 or so ohms listed for "H" then you know the sender is indeed either sensing a too hot engine or it is erroneously high for the gauge