When my 520i (M50) 1992 is at idle (hot or cold) it's at about 5650 rpm.. However, every couple of seconds or so there's a small bump/judder that can just about be felt through the car (floorpan/steering wheel etc). Very slight but any ideas what this could be? Changed the plugs, no different! an email reply would be cool, but any reply would be very helpful! thanks!