Well honestly your cooling system in it's stock form is not really adequate to handle the heat generated by open tracking. Folks who do it regularly have found it necessary to cut vents into the hood or box in the radiator to increase airflow. Airflow is the key to improving your heat extraction capabilities. Your thermostat does not keep going bad, the dealer is just applying the simplest solution to the problem. Likely a combination of one or both of these things is happening. First of all if your engine ever overheats and pukes out coolant it MUST be refilled properly every time, procedure here: Ford 4.6L DOHC Coolant Refill & Burp Procedure by Reische Performance Products Otherwise there will be air trapped in the system and you will overheat again shortly, as you've noticed. When the dealer swaps out the thermostat they also burp the system when they refill it which is probably what actually solves the problem. Second issue is there could be air getting into the system at the cap/reservoir seal. If you've puked out coolant numerous times the cap definitely needs to be replaced but possibly even the whole reservoir bottle. Very hot coolant puking out the overflow around the cap has been known to warp the threads and may prevent the cap from sealing properly over time. My thermostat may keep the car cooler the first few laps but will not solve your problem because it does not increase the system's maximum heat extraction capability. Replace the cap and reservoir to be safe and running the coolant a bit on the low side to allow additional coolant expansion is actually not a bad idea. Become familiar with the refill procedure and use it if you ever overheat again. You might consider a bit less boost for the track and also run very conservative timing in order to safely deal with the elevated temps. Your only other option is to find ways to increase airflow if you plan to track the car often.