Just to answer my own question, I started powering the DAQ with the DC jack, and it solved the horizontal lines issue. I still don't know what I did to change it, since the USB power was sufficient before. Now, the miniscope gets too hot to hold my finger on the PCB that the coaxial cable connects to for more than 5-10 seconds, and the top PCB gets pretty hot too. (The EWL side isn't as hot.) My hope is that the heat isn't enough to damage the miniscope or affect the animal.