Bert - thanks for the comments.

I'm not worried about spindle overheating as I'm only a small amount below its "official" minimum speed and the water-cooling keeps the spindle body at more-or-less ambient temperature even after 30-40 mins running. However, there are quite a few VFD settings which do things like boost voltage at lower speeds, and there are some torque-sensing features that might do something similar. However, if 400Hz is 24K RPM, then 5K RPM is still over 70Hz which seems to be reasonable for the VFD. I do need to have a look at the "torque" control settings on the VFD.

The effect I'm seeing is a bit like a governor on an old gas engine - the speed slowly drops until the governor detects that it has fallen too far, then there's a bit of a surge to bring the speed up again. In my case, I can't really detect speed changes, just this kind of "thump" which is a bit like a scaled-down motor-start effect. The VFD readout flickers between two similar readings, but it always does that anyway at any speed, and the difference between readings is very small. The speed displayed on the VFD is within a couple of per cent of the Mach3-demanded speed, which again is normal on my machine.

I'm using the router and spindle for something that neither was really intended to do so I wasn't really expecting anyone else to have experienced this. But it seemed to be worth asking the question!