96 S/320 100k miles. Lower gear, when I accelerate moderately, the engine speed increases in steps. Almost as if I was running an old carburator V-8 with mechanical secondaries. It's like a 3 phase acceleration while in the same gear. Crusing, the car doesn't display any problems. Gets around 20mpg at 60 - 65 mph. Any suggestions? Thought about fuel, already checked spark plugs. Thanks.

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1996 S320