Sanwa had one as well that did the same. I ran them for a while and never noticed. The issue is servo / pot (or encoder) hunting. It's very common in industrial servo systems (CNC machines etc) where by the servo doesn't ever manage to hit true "zero" when not loaded.
For example it's +0.001 when it's aim is 0.000. It tries to move to 0.000, but instead over shoots and ends at -0.001, so then it tries again to get to 0, guess what, over shoots and is back at +0.001 again! It keeps doing this and hunting around the centre point.
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Matthew White
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