I have seen many things set this off. The simplest were throttle linkage adjustments, but the fact it happens at speed probably eliminates that. Another common problem is the switch in the gas pedal (if it has one.. I always have to check the wiring diagram to be sure). I have also seen the alternator cause it and of course that car fits into the years where wiring harnesses deteriation could duplicate the problems of the systems they connect.

And then as stated above the throttle assemblies (2 on a 600) are the major cause.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician