I have been having the same problem from time to time. After I turn the car off and wait a while it seems to clear as well.
However tonight I figured out somewhat of a hack to get around it.
hold down the menu+map buttons and a screen adjustment menu will pop up, when I backed out of that screen I was able to hit more than one button in a screen with the touch screen failing. However when I went to my dvd rom audio playback menu, i could only touch the screen once before failing. This seems to be software in origin than hardware.
Let me know if this works on some level.
