Intermitttent violent shaking, at all speeds
 

I've got a 1998 Audi a4 quattro with 108,000 miles, just out of extended warranty (of course), and it has started violently shaking (felt like it was going to explode or at least unsafe to drive). This can happen suddenly while I'm idling at a stop light, or suddenly while I'm driving down the highway at 65mph, or at anytime in between. The check engine light first goes on, then as the shaking very quickly goes to severe, the light flashes. I can keep driving and within a few minutes it will all stop and go back to normal. Or, it will stop after I've stopped, turned off the engine, and restarted the car. There is no one situation where this happens. The local Audi dealership first said I needed a new ignition wire, replaced it, said it was fine, I got the car back and within hours it started again. They have now had the car for almost two weeks, (I've got a loaner), and they have no idea what the problem is. The service guy even drove the car himself for a few days with his analyzer with him, because at first they couldn't get it to happen again. Finally, it did, and they just have no idea what it is. Today, they said it was the computer (?) and they want to replace that for about $1000. Help!!!

I meant, they put in a new spark plug wire . . .