My 82 300D did the same thing. I found that if I straightened the cable right behind the speedo and pushed the cable down and through the firewall, the problem went away. I also added a little lubricant at the speedo end inside the cable cover. Has performed to spec ever since.

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Simon Shearer

1986 420SEL

2000 Porsche 911 Millenium Edition Carrera 4
2010 Lexus RX 350
2002 Kia Rio
2002 Hyundai Accent