I've been reading this thread for wot seems like an age. You poor bar-steward! We all know the feelin' of thinking you've solved the problem only to find out u havent! Ask Leslie Ash!!
I really wanted to say that i had a 325-Beemer that had this exact problem, it would also sumtimes hunt whilst trying to idle and then conk out when the revs dropped down. It was fine when driving, only cocking up when crawling in traffic. (Loadsa nocking it into neutral, then left-foot-brakin while holding the revs up wiv the right foot), I cured it by replacing the coils, odd i know, but it turned out 'no4' was fOked!! I know it's a Zimmer but it IS a straight-6 engine aint it! And you do seem to av tried everything else! See wot 'appens mate innit?? :)