280 SEL 4.5 running on 4 cylinders
 

My 280 sel 4.5 will be running along fine and then, suddenly it's hitting on what seems like 4 cylinders - This rarely happens - and I never know when, so it's pretty disconcerting - I can't depend on the car!

Any ideas? my mechanic thinks perhaps the trigger points of the injectors.

Please advise!

Before I even read your post I was tempted to suggest trigger points. Does it ever do it at idle? If so you can use a long screw driver placed against each injector and against your ear to listen to the click (or lack of) on each injector. Injectors work in 4 groups of 2. If one pair stop working then you either have bad trigger points or a fault in the D-Jet ECU.

Many times the fuel injector trigger points will get a coating of oil on them from excessive crankcase vapors, then they won't allow dwell signal. TRY cleaning them first with contact cleaner. OF course the complete ignition distriburor must be removed first.

Does this model have dual ignition points?
 

Some V8 Maserati motors had a Bosch distributor supposedly shared with certain pre-electronic Mercedes V8's -- perhaps just the big 6.3s (others had dual point Marelli distributors), and these were known to occasionally crack the wire which is attached to the points, causing a loss of spark to the four cylinders controlled by that set of points. This is less likely than the injection problem mentioned by the previous posters, who have genuine M-B experience, but it might be worth your time to check the continuity of the wires in question anyway,.