09 cobalt engine doesn't cool properly

My 2009 Chevy cobalt had its radiator bent by a rock 3 years ago. After that,
if I drove it in hot weather in the city and shut it off, when I trip to turn it on, it
says engine power reduced and won't start. It does turn over.  I have had my
radiator fixed, but the issue remains. If I'm driving on the high way, it's fine.
Also it's fine during the winter. I can feel the heat coming off of the engine
when it happens.

Any ideas?