Automatic Transmission Park/neutral Start Switch problems of the 2000 Hyundai Elantra

I t started around October 25,2001,while driving on the expressway. I was driving in 5th gear around 65 mph when suddenly the stick jumped out of gear into neutral by its self, I looked down to see what had happened,. Ther e was nothing attached to the stick, in other words nothing was hanging (no garbage bag or anything. This has happened at least 6 times. The car growls and I have to put it back in gear, now I'm anervous wreck. I took it into the the dealer twice and they couldn't find the problem. It has happened 3 more times since. I'm afraid to drive past 40mph or take the expressway. It seems to be a problem with the gear-shift\gas pedal\cluth, you can feel when its going to happen because th pedal moves under your foot and the shift gear moves if you put your hand on it.