Just an update, it turned out to be the fuel pump relay. I heard this happens to be a common problem with the w124 and bought a relay about 6 years ago and finally had the chance to use it. So far it has been just over one month and no further problems.

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1999 SL500 87K
1994 E320 112K
1998 Ford Ranger - the daily driver 274K and no major repairs (an indestructible truck)
1978 Pontiac Trans Am 110K
1977 Pontiac Trans Am 130K
1999 SL500 87K (the garage queen, only out in the summer)