You need to make sure the cooling system is working correctly, before you start tearing into it or you may never find the problem. Get yourself an IR temp gun and shoot the thermostat area. Shoot the upper hose going into the radiator and the lower hose coming out. You need to see a big temperature drop at the bottom hose compared to the top. If not, then it's possible the aluminum heads are shedding too much heat for the radiator to handle. But you won't know that without diagnosis and no matter how many times you snatch it apart and put it back together, you'll never find it IF that's what it is, UNLESS you actually DIAGNOSE something. You can slap all the parts at it in the world and you may never fix it. The timing was a suggestion since it is both EASY and FREE to try. Get the IR gun and get some temp measurements. You should already have one anyway. Best 20 bucks you'll ever spend.