I've had a rash of problems with this car lately. As per my other post, I've got the car idling good now. So, now I'm back to the original problem.

Under a certain engine load the car shakes (vibrates). Most noticeably on the highway when accelerating. It does not shake under normal driving conditions.

If I accelerate gently, it's fine. If I push it down a little hard, it's fine. If I accelerate "normally" then the car will shake (vibrate). I thought it was the tranny - but the trans shop said it was the engine.

What would cause a car to 'shake' under mild acceleration on the highway?