*UPDATE*

Turns out, it all goes back to those crappy Denso plugs...
Now when a plug fails, there's enough energy to fire it most of the time.
If you don't catch it quickly enough, it wrecks the coil and eventually the ECM.
(rotors were much cheaper :-) )

A quick swap to Autolites, a few coils and 1 ECM, and all units running well.

Thanks to all that responded.