Yes, that's encouraging. We now know that you are, in fact, getting to the ROM, at least to start with, but this leaves the question of why you're subsequently seeing no data or address activity. When this happens, are you still getting the MREQ and RD signals, or do they freeze as well?
You mention that you have the 813 diagram and manual, but another thing you'll certainly want is the processor manual, which will show you what the signals should look like. If you don't already have it, this is available at
http://www.nascomhomepage.com/pdf/z80-mostek.pdf