My vehicle suddenly started shaking or vibrating recently. When I got on the freeway it acted like it wouldn't go over 50 mph. We immediately took it in to a dealer and they haven't been able to figure out what is going on. In fact, now it hardly runs at all and two dealers can't figure out what is wrong. Any ideas? It had run for a while without a gas cap and they are trying to blame it on that but I've read that doesn't hurt it.