I've had that in all my cars with B230 and something different fixed it in each one. Try removing injector wires while it's running, if one doesn't change the way it's idling then it could be a bad injector. The 1990 740 has onboard diagnostics, there may be a stored intermittent code that will give you a clue. A throttle position switch and throttle plate that's out of adjustment wille certainly cause a rough idle. On one car the valves were so out of spec they couldn't be adjusted and I ended up doing a valve job. Also if the cam timing is off by a notch or two it will cause a rough idle. Put a vaccuum guage on the engine and see what it's doing. Check the ignition timing as well. Does your car have the correct air mass meter. I've seen people put a 007 AMM in a newer car and it throws the idle mix way off.