I got a makerfarm 12 inch. With rumba board. Got it running with a bulldog lite. I had have extruder problems, and clogs and such. After breaking 2 heat breakers on my hexagon hot end, I switched to E3D v6 with bowden setup so I can by spare parts if I break again. And I did break another heat break.

To the point, I notice that at slow speeds, such as when placing raft, the exturder skips. Often retracting the filament by quite a bit. I suspect this caused my last clog with my bowden tube.
I did a test, on my graphics display connected to my rumba board, I move 1mm on the extruder to push out filiment, and pinch it with my fingers so it has to pull on the filament harder. All is well. Then if i move the filament by .1mm and pinch, is starts skipping and retracting filiment. It did this without pinching.

Could this have anything to do with the current in the step motor driver?