02 Tahoe Thermostat Level Fluctuates

When the engine reaches operating temperature the thermostat opens and coolant will flow to water pump. If t-stat is bad, it can either be stuck open or stuck closed. If it's stuck open, engine will take longer to reach operating temp (and computer remains in open loop until it does), if it's stuck closed you'll start overheating shortly after engine reaches operating temp. It sounds from your description you run hotter as you slow down or stop, but stay relatively cool at highway speeds. That may indicate a problem with coolant flow - which can be affected by: cooling fans (make sure they are turning on), water pump (could be worn), radiator (could be partially plugged), radiator cap (not holding proper pressure), or a bad t-stat (not staying open when it should). I'd say your cooling system is in need of servicing - drain and flush, replace t-stat, replace radiator cap, check radiator for flow or the very least use an IR thermometer on it when it's hot, to make sure the entire surface is hot and no dead spots. Spotting a weak or bad water pump is a bit of an art form, if none of the above improve flow then replace the pump. Thermostats and rad caps are cheap parts, less than $5-10, and it'll cost you a couple gallons of coolant to replace them, so do that at least (as well as check fans) and see how it runs.

Thanka for your help. Can it possibly be a dash switch??