I think it is high speed, but it could be high rev. I have not experienced the behavior below 80 mph, although I have had the engine revved relatively high once or twice below that speed.

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Ben

The MBs:
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K