Ok, I have a straight-six who's radiator has no shroud (yet). The thermostat on the engine is set to 160 (don't remember why).

Now, here's the question:
If the thermostat is set to 160, that means that when it gets to 180ish it'll release the coolant from the radiator. If the engine is running at a steady 190 (since it's gotten hotter outside), does that mean that the engine is pulling the coolant constantly, not allowing it time to actually cool down?

 

Most likely the radiator is restricted with calcium. This restricts the flow which actually makes the radiator effectively smaller the more it's restricted. If the coolant was flowing too fast, it actually wouldn't be in the engine long enough to get to 190 degrees.

If it is indeed a radiator problem, the only real fix is to rod it out or replace it. Flushing is preventive maintenance - after a problem occurs, it's too late. Pull the cap and lower the coolant enough to see the few visible tubes inside the radiator. Any white or yellowish crud that doesn't belong is a sure sign of a potential problem. What you see is only a smidgeon compared to what can be built up in each tube which is around 2 feet long.

Also, with thermostats (all parts, actually) "new" doesn't mean "good". Quality control these days doesn't exist.

A loose fan belt or water pump impeller damage can cause this in a stretch, but they're way down on my list of suspects. Be sure your gauge is right then pick on the radiator.

 

Factory spec on these was 190°. It'll run better there, both power and performance, and in cold weather you'll actually get heat and defog.