If you have the faulty address & bits, you can pinpoint the faulty module. I don't have the schematics with me, but afair it's pretty straightforward. The first 4 MB bank sits on the highest address and memory grows downward. Within each bank the words are split into 8 ZIPs, 4 bit each starting with the highest significant bits.
Testing single ZIPs requires (very) special hardware and is not practical. By giving a good guess (see above) and swapping two modules for testing it's rather easy to isolate the bad chip.
If you've just upgraded the RAM and it's starting to get funny, there's a chance that the board is at fault. Mine lacked solder on two address lines, causing very weird problems.
