There is a known issue with Serato DJ and the Numark V7, NS7, and NS7II where the playhead can jump or skip around during some strange situations. Mainly when the motor is moving from one state to another (ie. Motor Off to On, Sync Off to On, SHIFT and adjusting Pitch).
This is because the platter is being ramped up/down and the MIDI messages can be confused. If you are having the track jump or skip around unexpectedly, then you are most likely experiencing this issue - especially if you are using things like Motor On/Off, or Sync On/Off.
To avoid this, try to keep the Motors or Sync either on or off throughout the duration of your set, this will help eliminate the chance of this issue happening.