I've replaced the 138 demux with a new 74LS138 and have the same issue with the ROM test. When the machine works (usually when it's cold and only for a few minutes) the CE pin on the ROM has 0.9 volts (low) and when the ROM test happens the voltage is 1.5v. The same can be seen on the OS pin of the MMU.
Not sure what to try next?
Thinking to get a new MMU from Best Electronics cause they're pretty inexpensive and either way it may be nice to have a spare. Also, the traces between the MMU and ROM look good and test fine for continuity.