I had done a reflash prior to writing this post. Also tried deadbands, but these spikes were way more than a small tolerance of <5% resolution. Did numerous power cycles as well.
I was working on my desk, two big LCD monitors and a bunch of USB, but no wireless... unless it's my cell phone; that's always on WIFI, but even that's on 5GHz. I do have a spare phone on 2.4GHz, so maybe that's the culprit.
I'm building another quad at the moment, my most deliberate micro build yet, fodder for "Show Us Your Builds". I'll be looking in this more, but the thread's at least raised my awareness of ambient transmissions.