Hey, 'Killer, it might not be the thermostat at all! With the symptoms you describe, I'd first look at the carbs suffering from the dreaded SU vapor lock. The give away is in your description when you indicate that the car idles a bit rough when the weather is warm and dies out completely when exiting a high-rpm run on the freeway. It seems to me that the low speed roughness is due to the temp of the carb set-up getting just high enough to begin to vaporize the fuel, but not enough to cause the engine to poop out. I would bet that once the car gets moving again, it settles down and runs pretty well. This is due, in part, to the fact that you now have fresh, cool fuel flowing into the intake. The stall out on the exit ramps makes even more sense along these lines when you think about what is happening with the exhaust manifold when you begin to reduce speed from freeway velocities....there isn't enough airflow to keep the whole thing cool, all that heat from the manifold now radiates up to the carbs, the float bowls, and the fuel lines, REALLY boils out the fuel and you have a recipe for instant vapor lock. It really doesn't matter that the temp gauge is showing cool....in fact, that's good to a point. What matters is the fact that the EXTERNAL heat sources (ie, the manifold) are really working overtime to vaporize the fuel. I would bet that after a high rpm stall, such as you describe, waiting a few minutes will either allow the engine to restart....or, if it doesn't stall completely, pumping the gas for a few minutes to keep the idle up will 'straighten things up.' Right? Accordingly, you might try to insulate the carbs and the lines a bit from the manifold heat...and for heaven's sake back off on the advance a bit. You might just be superheating the exhaust manifold and not realizing it. I had the same problem with my dual carb MGB and found that to be the problem. Good luck.

Brad