What's your tach read at 70mph? The reason I ask is, on the turbos, that's about the max out for third gear. Is the car shifting into fourth? Perhaps your shift linkage is loose and you've actually been running the car in S (slope) while the lever appears to be in Drive, thus negating the fourth gear which would allow you to drive at speeds over 70. Maybe check this first and take a look at your RPMs before getting into anything else.

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1983 300CDT -- 177K