meal time, since I just drove 180km autobahn, I could unfortunately observe it very nicely: in irregular distances the car (98 c70, 2.5T) jerks while keeping the speed. sometimes a while not, then again partly clearly noticeable. there is a short jerk, I feel as if the performance is short-term lacks. but is only a subjective estimate. error codes are not in, was only read out recently, moreover the ignition in the workshop (for gas conversion) was controlled, everything is in order. I also exclude spark plugs, were made in the middle of 2007 new. I know to identify “rumbling” equals the search of the needle in the heap, can theoretically be anything, but event. someone has a starting point. mfg