A laser type temp scanner can be a big help when diagnosing cooling system problems, and they're not expensive. Check Ebay for some possibilities for that option. What degree thermostat are you running, and have you "boil-tested" it to see if it's operating properly? Suspend the thermostat in a pan of water on a stove and gently increase the temp while observing the stat. It should begin opening at or near its rated temp, and open fully at higher temps. Be sure the temp sensing part of the stat is facing toward the engine- - - -it's possible to install it upside down. Professional shops usually have something called a "block test kit" that looks for exhaust gas in the coolant from a leaky head gasket by drawing an air sample from the cooling system through a colored liquid. The blue liquid turns yellow from a small leak, and green from a big one. The check fluid sold by NAPA stores seems to work better than the similar stuff sold by O'Reilly's for some reason.

Use the laser thermometer to check areas of the block and head to see if there are any hot spots. If it's hotter near the bottom of the cylinders there's probably an accumulation of rust and crud in the bottom of the water jacket.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!