I did if fact change out the 2 pieces yesterday. In both pieces there were cracks at where the nipples left the main crossbars. It helped a little, slightly noticeable. Not there yet, my mechanic sent it out today to another shop, he is stumped as well. New plugs, wires, timing chain, (timing set of course). It seems that was the only noticeable vacuum leak. Now the oxygen sensor will be replaced, it connects at the exhaust and wires to an inboard computer under the passenger side inside cabin in the firewall. We're still working on it.....
 It's a long shot but I saw a case like yours one time that the mechanic that had done the engine couldn't get it to idle smooth.  Turned out to be a new rubber part.  On each side of the engine between the valve cover and the injectors is a rubber hose piece that has 4 niples that hook to the 4 injector shields. One of the niples was pluged up.  The engine gets idle air through these niples hince one cylender got no idle air and would miss.  If you take off the hoses and look in each niple you can see if it is blocked.
 This is a very hard problem to diagnose, but an easy one to fix.
 Paul
   "Tony Pike" <tonypike@telus.net> wrote in message news:dsqSj.3729$XI1.2018@edtnps91...
   I just had a 1988 420 SEL  4.2 Litre motor completely rebuilt at a reputable shop. Everything seems fine, leak down test, wet and dry compression test -all checks out o.k. When at idle, the motor visibly shakes and seems to be missing. The mechanic is even at odds here, and might send it outside to determine the problem. Of course everything was installed correctly, timing set, plugs gapped, etc. Could this be a vacuum leak, fuel/air mixture issue or some relay at fault here? Any suggestions greatly appreciated.
       Tony.