[CAD_CAM_EDM_DRO]CNC Pulse jitter?

I have seen talk of computer pulse jitter causing axis drive problems
on stepper driven systems.
This may be a difficult question but, how much is bad enough to be
on the threshold of being a problem?
Is there a rule of thumb or easily perceptible symptom to look for?
What is the std method/spec to measure jitter?

I'm trying to troubleshoot a mill and apparently I have a bad case of
resonance but, in looking at the incoming pulse train on the scope
(Tek 2236) really makes me wonder of other bugs as well, not helping
the situation.
RichD