Hi Gerry, I remember getting this problem with our old AK-100's, it was a bugger to fix and I don't think I managed to put it down to any one thing. Can't say that i've seen it with the 200 or 200S. I remember struggling to get a machine to do it in the workshop, would only fail in the unit.
You say you can see the flow go from +-2 to +- 6ml/min, have you been able to log the variance figures (SRR 34,35,36) at taration and see them go outside the limts at zero or differential flow?
Fraser