Its not the valves. MB V8s do not burn valves.
It is very likely that you have a bad spark plug end (resistor). With a secondary ignition scope these are easy to pick out even when they aren't causing the misfire. This test is a must for all MB misfires as it is so quick and easy.
I sometimes get a real chuckle about all the plug wires, distributor caps, coils, etc. which are replaced to avoid paying a professional to do this test.
If I were going to do a shotgun operation on your problem I would start by replacing all the plug ends. Leave the wires and their factory routing alone as they almost never go bad. The ends should be available here or elsewhere for around $10 a piece. Still more than a proper scope check, but my standard recommendation now after verifying and fixing the problem with one end, is to recommend replacing the rest so that they do not need a scope test to be found as they fail one at a time.
BTW, here is my technique for replacing the ends: I first take a rag and wrap it around the upper rubber coated end of the resistor. I then take a pair of pliers (visegrips usually) and I squeeze closest to the end of the ceramic under the rubber, till it breaks. Since this is under the rag and the rubber the sharp pieces of the broken ceramic are contained. Next I slit the rubber all the way to the plug wire and pour out the broken pieces (carefull they are sharp). The point of all this is to be able to expose the end of the plug wire which has a threaded metal cap made to it. Once exposed it can be easily turned to unthread the remaining resistor body. If one tries to unthread the cable from the end without doing this, some portion of the wires will separate from that threaded metal cap requiring a new wire or a separate repair (the caps are available separately but replacement is much less fun).
On reassembly, a silicone type rubber lubricant eases the insertion of the wire into the plug end. Hand tighten and you are good for another 100k.
Bosch Master, ASE Master, L1
33 years MB technician