I had a SOHC Expl that would push lots of coolant into the overflow tank once in a while. I knew it had a head gasket exhaust leak, because it would blow bubbles into the overflow tank. I found that if I let the engine warm up idling in my driveway it would fill up the overflow tank and overheat. If I just jumped in, started the engine and drove off it would be fine, never overheating or filling the overflow tank with coolant. I guess heating my engine up quickly blocked off the head gasket leak with the metal parts expanding where allowing it to warm up slowly while idling allowed exhaust to displace the coolant and push it out. Anyway, my point is have you considered that you might have a head gasket problem? You've pretty much changed everything else. I was just thinking maybe you had a sticking t-stat, but I re-read your thread and you say you've changed it twice already. What brand of t-stat did you install? I had a problem with aftermarket t-stat and returned if you a Motorcraft. Other than my above thoughts, I'm stumped.