It is quite possible that those are sampling steps as you stated. If you look at the Nano 50Khz square wave display (attached) you will see 25 steps in the display during one sample time = 1us. This square wave on the Nano is a similar percentage of sample rate as the 3.55Mhz on the Quad. Maybe BenF could comment on the reason for these 25 steps in the Nano. I suspect that they are display update steps. Could the Quad processor be running fast enough that you can’t see it’s display update steps?
I know about the interleave display issues and I am trying to keep this thread on bandwidth topic, that is why that info and associated pictures were not included.
I am going to hold off on the soldering iron while I conduct further testing on the factory issued Quad. 
