Remember, the thermostat doesn't really control if it cools or not, it just controls at what temp the engine runs at. Your cooling system cools.
If you are overheating or running hot, make sure you ARE running hot. Is the temp gauge right? If so, as someone else said, check your flow (water pump). How old is the radiator? Is it clogged? Fan shroud on? How are the fins? I do my mix stronger than 50/50.
I've got a friend that can't understand why his rig runs hot... he has so much crap in front of his rat it is amazing. Bumper, air horns with compressor, this, that... gee, it runs hot, wonder why...
All that said, years ago I used to run a cooler thermostat and hotter plugs in the summer, then change back in the winter. Once a year I did a total flush. That was back when I had money.
I also have a electric fan in front for when I rock craw. Then again, I also have more than one heater core and fan. (got tired of the wife saying she was cold, now I can roast her)