2003 Hyundai Accent 2003 Hyundai Accent Shakes over 70mph

I have a 2003 Hyundai Accent with 80,500 miles. One day I drove it from NJ to Maine. As soon as I got to my destination in Maine, the car died. I drifted to the side of the road and had it towed to a local Pep Boys. Pep Boys ran a diagnostic and found nothing wrong with the starter, alternator, or battery. (The battery was just replaced 12/06). In fact, when the guys at Pep Boys got in the car the next day, it started up without a problem. So I took the car and drove it back to NJ.

As soon as I arrived to my destination in NJ, same problem happened--car just died. I had it towed again, and the mechanic replaced the crankshaft position sensor. When I got it back from him, the check engine light started coming on randomly. I called him, but he said the two couldn't be related and to bring it in so he could diagnosis the new problem. I didn't really trust him from a previous experience so I decided against taking it back to him.

I took it to PA where my parents live and took the car to a Hyundai dealership. They ran a diagnostic test and said it needed a tune-up, as it was only firing on 2 cylinders (it is a 4 cylinder). After the tune-up, nothing was fixed (although I'm sure the car was healthier!). The car continues to have problems, specifically, I can't drive over 70mph without the check engine light flashing at me. Once the car hits 70mph, the steering wheel starts shaking and the check engine light flashes until I decelerate. Once I'm beneath 70mph again, the car returns to its normal habits.

Does anyone have any idea what it could be? Any help would be greatly appreciated. Thank you!