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That would be a problem with control circuitry. Either the motor controller board for the incline motor, or else the console, is defective somehow. That anything moves when the safety key is not in, is a sign that you could eventually get into rather dangerous territory with this problem, so I advise you to get it fixed or else get rid of the machine (and, not under any false pretences about what's wrong with it or whether it is working.) The track is not supposed to move when the safety key is out, period, and injuries could result.
Personally, I would guess that the motor controller board is defective, and is 'taking actions' on its own.
Speed defects might be anything - even a wrong voltage from the power supply. But more likely either a control board, or the console, or doubtfully the motion sensor though that is unlikely since it merely senses per-revolution of parts of the machine, and it is the console or control board that counts the revolutions and sums them up to a speed. ...or, controls the system at a certain speed.
Summing that up, the motor control board would seem the likeliest candidate for replacement.