Chevrolet: 1998Chevy Venture Van, cv joints, venture van

Question
Sparodic bucking. One to three rapid occurrances while cruising. Its a metal on metal lurch forward clunk and a lurch back very fast and someone may at first think it was a hole in the road. Similar to really bad plugs going up a hill but its the metal sound thats disturbing and can occur on a flat, hill, or decent.

The transmission goes smoothly thru the gears but it feels like  gears tooth metal on metal. sound. I am not shifting (Auto Trans) thru gears when this occurs.

CV joints mb. i.e. lurching  

If I took it to a shop it might not occur during a test drive however it would almost certainly come up on a second or third test drive.

Thanks  

Answer
all i can think of w/o driving it myself is motor mounts and or tranny mounts, also how is the drive train, if you put the car in drive and place your foot on the brake and give it gas as this will at most times lift the engine up and the fan hits the rad. shroud and makes that type of sound. or get it up on a lift and check all items that might be loose ok