I realized after I posted about A1 that it doesn't quite make sense, because if your address lines were shorted, the computer wouldn't work at all. But if the A1 output from the video address generator were failing somehow, that might do it. On the Mac Plus, the video address comes from some counter chips and a buffer. I don't know about the SE/30-- probably it comes from one of the custom ASICs? Maybe the A1 trace to that ASIC was partly burned away by capacitor goo, or the chip itself has gone bad? Or if the chip is socketed (doubtful), maybe it needs the pins cleaned and resocketed?

Sorry I don't know exactly where to look. I dimly recall seeing that someone posted SE/30 schematics on another thread not long ago, though.