| Engine speed is governed rather then actual vehicle speed. As you know due to the final gearing the engines rev high on the highway even at regular speeds.
Questions: what was the RPM at 90? Why did you feel the need for an Italian tune-up, how was the engine running before? How many miles on the fuel filters, air filter and tank screen. Was there excessive smoke? Motor mounts may be an issue as well.
Depending on the miles and over all condition of the engine (compression in particular) 90 may be her top speed.
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1991 300 SE (w126.024/M103.981) 140,000 miles
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