what makes temp guage bounce around

Not sure what else it could be other than an electrical malfunction if you are sure that the vehicle is in fact not over heating and running at normal temperature. 422,000! Wow. You must really have pampered that machine. Thats amazing.

change out the thermostat...easy fix.

A broken thermostat wouldn't cause the gauge to bounce around......... The water temp sensor is probably dead or dieing.  Find out where it is and replace it.

Do this test:  Ground out the thermostat lead.  If the gauge goes to max, it's the sending unit.  If it does nothing, the issue is the gauge or wiring.

WOW 422 000 YOU SHOULD GET A HOLD OF GM AND SEE IF TH NEED A MUSEUM PEICE TO SHOW OFF THE LONGIVITY OF THE S10 LOL I WOULD DO EXACTLY WHAT BUDDY SAID ABOUT GROUNDING OUT THE UNIT HES RIGHT BUT GM MECHANICAL GUAGES ALWAYS BOUNCE AROUND ESPECIALLY WHEN THY GET OLDER AND HAVE LOTS OF MILES ON IT YOULL PROBALLY END UP CHANGING TE TEMP SENSOR AND GAUGE

well I doubt its the timing. Could be possible that you have a clogged radiator, or a sediment built up in the engine block