Best explained by looking at the log file.
RPM and cam positions(indicated only) jump around when under load.
I thought it might be noise, It's not a RF noise issue, eliminated that by running a separate shielded REF cable to the ecu along a different route to the rest of the loom.
I thought it might be voltage drop, eliminated that by powering coils and injectors from a separate power source, relieving the load on the ECU power source - solved the voltage drop at load, but no other difference.
Engine is Subaru EZ30R - setup using mode 39, as I can't get mode 52 to fire at all.
Tempted to pull the M600 and sell it on ebay.
I invite you to restore my faith.