That's quite a collection of symptoms, but I don't think they're connected, unless the immobilizer modifications have totally screwed up the electrical circuits. The sudden engine cutoff at highway speeds suggests an interruption of the fuel pump electrical supply. That would lead me to OVP, fuel pump relay (or MAS, if so equipped), and engine speed sensor (TDC signal). ABS light coming on during these episodes?

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't