Spark plugs wouldn't cause intermittent problems, forget that.
The electric fuel pump is powered by a relay (a big switch). These fuel
pump relays get tired over time and become intermittent, more so with
time. The curse of intermittent problems is that they can't be
duplicated so the only course of action is to replace what one believes
is the most likely suspect, in this case the fuel pump relay.
There's also another possibility, it's the over voltage protection
relay. Others can tell you more about it, I know only that it can cause
strange engine behavior.
Final suggestion, find an established independent M-B shop. Dealer
service is too expensive.