These Mazda lumps tend strongly toward mystery overheating when the HG fails too. It's an easy thing to test anyway, I like to use those coolant hydrocarbon testers with the indicator fluid but just starting it up with the rad cap off will tell you right away if it's really bad.
A PO may have used stop-leak too. Having a layer of wax through your water jackets in Texas heat would definitely cause issues.
Also, I don't mean to sound like a jerk but it sounds like you're engaging in shotgun diagnostics. All the parts you've replaced that didn't fix the problem were pretty easily testable. You'll save yourself a lot of money if you do a little poking around before ordering parts.

Last edited by Tommychu; 06/08/15 05:39 PM.