[W126 Coupe] Tune up woes

[Dick Spellman]

A fellow SEC type with a 500 engine had the work performed to fix a backfire problem by a local (non-MB) shop.  They installed new wire set (the brand and set-up are just fine), a new cap and rotor (both mongrel aftermarket not Bosch) and new plugs (my friend thinks they have 2 electrodes or more).

Anyway, the car starts and runs fine but, when on the highway doing 70 or so the engine just quits momentarily and then kicks back in.  This all started 1,000 miles after the tune-up. 

What are your thoughts on a path to diagnosing the problem? Regards,

Dick
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