So after doing some reading on the subject, it looks like the issue is "oversynchronization", as per the image on page 36 of this book:
http://www.tubebooks.org/Books/smith_oscilloscope.pdf
I'm going to have to dig back into it and figure out why that's happening. At least I have the EICO 425 on my side now, to assist with troubleshooting.
I read another document by tektronix, about oscilloscope troubleshooting, and it says to isolate the problem with the cover on - good advice I think, I never recap before troubleshooting, but this one I tried to do a quick fix, and ended up just knocking it out of calibration.