First, flushing the radiator might not be enough, it may need to be rodded out to clear out clogged tubes that the flush won't touch. Next, how did taking the center out of the thermostat work? Usually if a bad thermostat is causing one to overheat it's because it's stuck shut so no coolant can flow. Taking one apart, or removing it will usually cause the coolant to flow too fast (((not hanging around long enough to pick up heat like it needs to in the engine and flowing through the radiator too fast to transfer enough heat out of it))) again casing an overheating problem. Another thing I haven't seen personally, but have heard about on several occasions, is a bad water pump. Even though it checks good (ie-the impeller is tight on the shaft) when you look at it, once the system heats up the impeller will slip causing a reduction in coolant flow and an overheating problem. You also said it had been overhauled and the problem started shortly thereafter. If your engine has precombustion chambers it's possible one of them didn't seal and combustion gasses are entering the coolant system that that way. If that's the case eventually it will erode away at the metal around the leak and you'll start seeing coolant getting into the engine when it's not running, but as long as it is running combustion chamber pressures are high enough to keep the coolant out. The same could also be happening due to a bad head gasket but that will typically allow the coolant into the engine much easier than the lower seal on a precombustion chamber will so should be much easier to diagnose as being the problem.
Beyond that I really don't know what else you can check, or do as you've already covered most of the basic checks already. Good luck and keep us informed.