Where does the vibration come from, the steering wheel, the seat, or from behind you? The steering wheel would be an indication of the tires or front hubs, the seat would be a drive-line issue, and if it feels like its comming from behind you it woul be something in the rear axle. I would think if it is anything with the front susp/tires/hubs, the driveline or teh rear it should happen all of the time, not intermitantly.

A few years ago I my 3.0 would kind of shudder, though never violently enough to make me need to pull over, just the CEl owuld come on and then go off. The ECU registered it as a distributor code, the dealer diagnised it as a burnt valve. I rebuilt the top end w/o replacing the distributor and haven't had the shudder since.