1996 GMC Jimmy temperature fluxuation

In slower traffic (in town), the temp gauge sometimes jumps up into the red zone (causing the "check gauges" light to come on) within one to three minutes. I pulled the thermostate and tested it in 195 degree water to see if it opened properly and it did. I did notice the water looks rusty. I recently had a heater core unit installed but they must not have drained the system. This is also something I probably should do but cant find the drain plug in the radiator.
The other options may be the heat sending unit or the clutch fan but not sure how to diagnose them.
What's your take?