This sounds like the issue should be fixed inside xwax or do we introduce the jitter by a not ideal usage of waxx? 
You should ask xwax developers, maybe they have good hint. 

In the meanwhile a short integrating IIR that does not put much latency to the scratching might help. I had good results introducing it for Mouse Snatch. src/engine/positionscratchcontroller.cpp. In any case I would introduce an additional low pass filter (long IIR or mean value ring) for the BPM display. You are right, No one will like a jumping display.