[SoCal LS-L]

Hmmm well since it doesnt seem like the resistance from the coolant temp sending unit would change like that..... I would feed the gauge power from somewhere else... maybe even its own lead from the fuse box. It uses supplied power to check the resistance from the sender, so thats probably why your reading changes when you use your accessories. I would also maybe try a different ground, it can have the same effect if those systems use the same ground.