The elusive front-end vibration.
I have a 1984 AMC Eagle. When driving on a paved road, the front end will sometimes start to shake at around 45 mph. You can see the dash vibrate! I can go for months or over a thousand miles between incidents, and if I stop and then start again, it runs fine. My mechanic cannot understand it either.
TOM: This is a tough one, Robert. We assume that your mechanic has checked the obvious stuff, like every single piece in the front end. Bad ball joints or tie rod ends, a worn idler arm or pitman arm, or even a loose steering box could make weird stuff like this happen.
RAY: But I'm going to suggest you take it to another mechanic for a second opinion. Based on my experience, it sounds like a bad ball joint. If your mechanic either didn't think to check the ball joints, or checked them and missed a bad one, he could be leaving you in a very dangerous driving situation. Ball joints keep the wheels from falling off, and driving with one wheel tucked under the car is not a good idea, even in a vehicle as technically sophisticated as this AMC of yours.
TOM: If the front end really does check out, you might look for a bad universal joint on one of the drive shafts, or perhaps even an intermittently sticking brake caliper on one of the front wheels.
RAY: Those last two theories are a bit far fetched, Robert, but since you drive an AMC, you're obviously used to things that are far fetched. Best of luck, and get it looked at soon. It could be something dangerous!