There is another active thread that I am repeating this from - it is someone else's idea, but it sounds like it could possibly be your problem, and its easy to test.

Perhaps your fuel tank cap is not vented properly - that is, the vent hole is plugged. Should be easy to test. Next time you pull off the highway from a high-speed run, jump out and take off the "gas" cap. If it runs properly immediately after taking off the tank cap, the vent must really be blocked.

Would be interested to know if this turns out to be your problem.

Ken300D