Questions about ATX spec. PSU's ripple may be out of spec?

I've been wondering if my PSU (Corsair HX750) could be the root cause of two USB host controllers intermittently crashing. To diagnose I've been monitoring the power rails under load, checking for dropouts or large ripple ( which I might have on the +12V rail.) I found a reference to the ATX spec that said 120mV allowable ripple on the +12, but couldn't find out if this was only for a specific load, or no/any load.

So my question is how is the ATX ripple spec defined? Does anyone know if there is a specification for under load? Is the spec the absolute allowable under any condition? And more importantly, where in the source->distribution-> load network is considered a valid test point? Because I imagine it will vary considerably, and a system showing <100mV right at the output connector could also easily show >500mV on the end of a cable serving some RGB case lights.

For the record I measured about 350mV ripple, but this was on the splice end of a long case power cable, and also using some cheap probes which I'm sure contributed greatly. But before I break out the proper equipment I want to get a better idea what I should be looking for.

Thanks!