7i76e/8i20 losing spindle speed.

From time to time I notice some strange behavior with my setup and I'm not sure exactly where it's coming from. The spindle will run fine @ around 2500rpms at whatever current level it takes to maintain that speed. At some point after running for a while it will only be able to maintain ~1700rpms and will ramp current up to the full PID output. I'm not sure if it's a PID setting or some software glitch or the servo rate causes it to not be able to maintain commutation. I can stop and restart the spindle fine but it will still be limited to the reduced speed and maxes out the current. All I have to do to get it working fine again is restart LinuxCNC. Any ideas where to start looking?

What BLDC component commutation method are you using?

If you are using quadrature, you may be losing counts which would cause commutation issues.

I'm using "qh". I'm not sure that I'd be losing counts unless it's a servo thread thing, that I suppose is possible. Because of the way the encoder is mounted it's possible that the encoder body itself can rotate a degree or two, I'm not sure if that effects the angle or something since qh also uses the hall sensors at least at startup.

For reference I use a 1000ppr quad/differential encoder @4000 counts/rev

I'm not sure but I think QH mode just uses the Hall effect signals for initial rotor position determination, so any lost encoder counts will cause a cumulative commutation loss

Is the encoder filter bit set true?
Also is the encoder jumpered for differential mode? (it will work jumpered for TTL mode with differential encoder but you will lose noise resistance)

Are you using a common mode choke (large ferrite bead) on the spindle motor U/V/W wires?

The hm2 encoder filter bit is set to false, not sure why I did that but it's written out so I must have done it on purpose

The encoder interface is set to differential on all channels.

I use 18awg shielded cable with a clamp on ferrite close to the motor. Shield is grounded one side.

I would definitely enable the encoder filter