99 GA Termostat Problem??

When i start my car on cold days after its sat for a while. The engine heats normally til it hits about 160 degrees. Its normal run temp is about 210 according to the gage. But once it hits about 160 it jumps up to about 260 for 10 seconds or so and then drops quickly back to about 180 before reaching its normal temp of 210.

Once that happens the car runs fine with no temp problems.

Is this a bad thermostat??
Is the bypass kicking the temp back down after 260??

or is it possible this is a computer problem with my gauges?

replace the thermostat

I too would say the thermostat , but you might also try checking for air in the system.

Theirs a bleeder vavle on top of the water pump.
Black tube,
loosen the smaller brass bolt slowly
about 1/2 - 3/4 turns and see if air doesnt purge out.