OVERHEAT HELP, please
 

I'm running a 406 sbc built by TriStar engines. It was dyno'd at 530hp and 525tq. I have a new three core aluminum radiator with Flex-a-lite 295 fans. I had a Mr. Gasket 180 high flow thermostat which I just swapped with a Stant 180 high flow (thought that might be my problem). I'm running a TH350 tranny with 2500 stall convertor.
I can let my truck idle all day long in my hot shop and it won't get over 180. I can drive it down the highway and it stays at 180. After driving down the highway for about 7 miles, when I turn down a slower street and run about 30mph, the temp shoots up quick to 230-240. When I shut it down and restart it, it cools right down, but seems to run a little hotter than it should. I found if I manually down shift it and keep the RPM up, it seems to help.
I am not running an auxiliary transmission cooler, just using the one in the radiator.