I had this problem with my gen 8 which I cured by putting a metal ball on the end of the needle bar as I use bishop grips and it's a large hole thru it and I worked out that as I moved the machine around the needle bar wasn't always hitting the cart plunger dead centre so sometimes it was pushing the plunger to the side wall of the grip causing more friction and making the machine fluctuate.this cured my problems and the machine doesn't miss a beat at any speed.