Latest up date on this 420 sel and its poor performance on occasion's, low and behold as stated by Ctaylor738, It was indeed a fuel supply issue! Not knowing at the time of this original posting, that there was a filter in the fuel distribution unit, and that this was the culprit of our loss of fuel supply and the reason for its bucking and loss of power. this was found after my son who has had this car for quite some time and had this same trouble several times until he finally told me about it. I was looking for the answer again, when I had sent the fuel distributor off of my 190E 2.6 out for repair, I found the filter, I then pulled the line from our 420 and removed the filter from it and the trouble has gone away. I hope this helps others who have had similar troubles. JNT.