Not sure what the problem is. After the car is driven for over 1/2 hour at highway speed, there is bumping and shaking as if you have a flat or unbalanced tire when you try to accelerate and you can't gain or keep speed, have to pull over and wait for a while. Then it'll happen again if you go highway speed for about 1/2 hour. Even while this is happening, it's fine coasting in gear (Auto tranny) or out and the engine sounds ok when you rev in neutral. SO I don't think its tire,suspension, engine or brake related. But the tranny seems to shift fine. The car can be used in stop & go traffic all day without a problem. What could this be? No mechanic has seen the problem cause you need 50 mile of open road & time to kill. Any ideas??