Normal cars can cruise in 5th gear at like...50mph. But not my car. When I put it in 5th, it does this harsh surging that just shakes the whole car. I can't figure out what it is. At first I thought it was something with the clutch and it just didn't like low RPM's. But it doesn't do this shaking in 4th at a low speed/low RPM. Just 5th gear... any suggestions as to what could be wrong? It's not a huge concern to me, but I'd at least like to know what is causing it.