For almost a decade, my 89 245 has had a rough idle. The choppiness seems to vary from moment to moment, but gets worse when the engine RPM settles down to idle speed (around 800). I still have the original 561 ECU in the car--could this be the source of the problem, or is ECU failure an all or nothing proposition? Could installing the newer 951 ECU offer performance improvements?

OR, do I just have consistently dirty fuel injectors (even after running on 93 octane fuel for years)?

(BTW, The usual other suspects have been ruled out--engine out of tune, dirty throttle body, bad sparks, vacuum leaks, etc.)

Thanks.