83,000 miles. The service light came on tonight. The speedometer showed 90 mph on 2000 Lincoln Navigator

I was only going 15 mph. Shifting seemed not good. Added 5 gallons premium fuel. Ran better. Speedometer acted normal. rpm jumped at times. light still on. running rough. made it home. Where do I start to fix this?

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2 answers
have it scan tested for codes and go from there
start by pulling code(s)