a simple RC filter 10K & 5UF was the magic combo for the ZR+ Index Pulse. Now the RPM Displayed in mach3 is correct and stable now at all speeds!
Well, the general idea of an RC filter is certainly right, but 10k and 5 uF give a time constant of 50 milliseconds. If those are the correct values, you should be getting an RPM reading of zero. Do you maybe mean 5 nF, or 5 pF? I use 10k and 150 pF myself.
Or do you mean a pull-up of 10k with 5 uF in parallel?

Cheers
Roger