This could be for any one of a number of reasons, though there are four which tend to take precedent over the others as being the most important or at the least the most common: there could be an issue with the speed selector switch, there might otherwise be an issue with the internal belt of the turntable, the turntable parts might be too dry and cause the turntable to spin too fast, or there might even be an issue with the motor's internal clock that needs calibrating.