Encoders counting..... sometimes

[Tom Line]

We purchased high speed encoders to measure rotational rate. However, we are seeing something I wouldn't expect.

The encoder is here:
ENA1J-B28-L00128
http://www.mouser.com/ProductDetail/...Ha2t9hjO c%3D

The encounter counts up and down correctly. We have power wired to +, ground wired to -, one signal on DIO 3 wired to A, and one signal on DIO 13 wired to B.

However, there appear to be 'dead spots' on the encoder. If we rotate the encoder slowly, we find locations where it starts counting up just as if it were unplugged.

What might be causing this behavior? We have 6 of them, and they all seem to act the same way. We have removed all the wiring and plugged them into the digital sidecar with short pigtails, so I'm fairly sure there isn't any wiring problem. Unfortunately the counting is in the FPGA, so I can't really see the code to debug what is causing it to count up.