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That could be either a marginally maladjusted or defective speed sensor in the inner workings of the treadmill, or else a control console or control circuitry defect.
How long does it take the treadmill to slip into a faster speed?
When it slips into a faster speed, does it do so gradually, or quite suddenly? and does it (at a guess) go twice as fast as set, when it does get faster?
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