Give this a try... pull the vacuum line from the shut off valve on the IP.
I had a similar issue with my 300SD where it would run fine and then slow down and die. It turned out to be the shut off switch in the dash leaking vacuum into the valve on the IP. Vacuum would slowly build up until it shut off the fuel rack.

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"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
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FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
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Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
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'92 Subaru
'96 Impala SS
'71 Buick GS conv
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