I agree Oldblue.
I still say to look 1st and verify that the fan is (or isn't) running. The sitting still or creeping along with no air flow would certainly cause the overheating that the OP has under those conditions.

 

x2 - check fan first, then thermostat.

If it was me, I would not drive it till it is checked out, and resolved. Having it hit 250F is often a death sentence for the head gasket/head. And that gets VERY expensive.

 

At those temps, the Dexcool will have crystallized, so flush it out and replace it and the thermostat and of course double check the fan

 

Hey folks. I need to make a slight correction to my statement that the SS cooling fan should run all the time.
This may not be entirely true. I cranked mine up just to check(mines been sitting about a week )
So it appears the fan did not turn on at startup. I engaged the AC and the fan did turn on as it is supposed to.
Turned off the AC, but the fan stayed on for about 2 minutes until I shut down the car.

I may dig into the particulars of "when" this cooling fan is supposed to run on the SS.