First, thoroughly clean and oil your machine. Remove the side panel from the front of the head and clean inside, then give all the moving parts oil. Remove the needle plate and clean underneath. Oil everywhere then let the machine run the warm up cycle.
If that doesn't help, run the needle speed auto-adjustment as shown in this video:
If the problem continues, check your motor brushes to see if they need to be replaced. Replacement motor brushes can be purchased from www.quilttech.com.