No, it wasn't a bad CPU.
No, it wasn't a bad TED.
No, it wasn't a bad PLA.
No, it wasn't a bad ROM chip.
In the end it turned out to be bad ram and a bad logic chip - 74LS139.
Now it is on to fix my Plus/4 which exhibits the same symptoms.
A side effect of this fix is that I was able to verify the remaining PLAs. CPUs and TED chips I have are all OK.