my 82 320is had the exact same problem about two months ago. after all the frustration i could stand, i took it to nicklaus autosport, which has built and tuned bmw race cars for 25 years. they adjusted the fuel mixture ratio, tuned the valves, and replaced the fuel filter. now it runs much, much smoother, but with 170000 mi. with no rebuild, it's still not quite how it should be. i believe a 2.1L bore with 9.5:1 2002tii mahle pistons, tii crank, 286 shriek cam, bmp race valve springs and ti retainers, and 1.5" header will solve this little dillema in a month or so :)