What makes a temp gauge bounce back and forth but doesnt overheat on a 1999 s10 changed temp sensor thermostat radiator cap and clutch on fan and flushed and changed coolant?

Make sure you have the correct thermostat in there, if it is set too high you will get that bouncing.

The thermostat is set to open at a certain temperature and allow the coolant to exchange with what is in the radiator. Under normal conditions you will notice a rise and fall of the temp gage as the coolant reaches max temp then lowers after the exchange.

An overly sensitive sensor or an aftermarket gage set up will exaggerate it.

I would not worry too much since it is a good indication that your cooling system is working well, the coolant in the radiator certainly seems to be cooling down enough to cause a big swing in temp.

Check on that thermostat though as it may be letting the coolant get too hot before releasing it.