thermostat jumps up and down after a flush

Ok. I'm confused. First you say the thermostat jumps, which is impossible if the housing is properly torqued, which it has to be if it isn't leaking. Then you say it's the temp gauge. Is it electric or mechanical? If it's electric make sure it's properly connected and tightened. Also make sure the ground to the gauge is tight. If it's mechanical, check the coil-covered probe wire for bends, kinks, and breaks.