After extensively wiggling the plug leads on #1, #2 and #4, the plugs leads came free. Boy, were they stuck ! It was the grip around the metal tip and not vacuum or anything else that caused the reluctance to unplug .... The new plugs also grip the plug leads with a vengeance ....
I also found the culprit responsible for the erratic idle.
It turned out to be the spark plugs on #1 and #2 cylinder. Both had the nose porcelain cracked, probably causing spark to be lost to earth. It also looked like the plugs had worn past their useful life (I was under the impression they were changed at 120 000 miles at the dealer - I did it now at 136 250 ).
Anyway, the engine now idles smooth as silk.
Thanks for all the replies. It sure did help a lot in preparing for the job ahead.
I'll keep the vacuum tracing tips filed ....