Xzero machine, tuned and running great for 3+ years now. Until very recently the following is happening

Note: Spindle is a 2.2 KW - 400Hz ER-20 collet, VFD is a 230V, 15A from the same mfg as far as I can tell - the common package available widely across the web.

Problem as of last week:

- loss of torque/power during cuts
- RPM is visually and audibly slower when engaging material - despite VFD readout remaining constant
- example cut parameters (3/8" upcut spiral, .125" pass depth, 16000 RPM, 70 IPM feed) *** these settings are an example of a VERY conservative cut for this machine, in actuality we've run the machine much faster than this

- I've gone through the VFD parameters and nothing is out of the ordinary there - nothing changed since setup
- Spindle is not overheating
- Just noticed that the fan on the VFD is no longer operating (could this be an internal heat issue? - perhaps there is an internal safety limiting torque upon fan failure?)

Has anyone experienced the above symptom(s)?
Can anyone suggest any troubleshooting steps... i.e. VFD voltage/amperage checks?
Is it possible there's a loose wire or something internal in the VFD that could be faulty, resulting in poor communication to/from the spindle?

Thank you!