Plugs didn't help, and now it seems more regular now and drives like a "lumpy V8" most of the time now instead of the few seconds it used to drop a cylinder....
During the painstaking process of changing the plugs, I quickly put the multimeter onto each coil and got:
Cylinder 1: 852 Ω
Cylinder 2: 903 Ω
Cylinder 3: 687 Ω
Cylinder 4: 809 Ω
Cylinder 5: 859 Ω
Cylinder 6: 851 Ω
Now, these figures seem very high compared to the series 1 RB25's, (0.7Ω is the norm?) but I believe my coil packs have the igniter built into them. (unlike the early RB25's which have the igniter inline before the coils)
I have a Japanese Engine manual, but couldn't find anywhere how to properly test these coils and what the normal values are.... Can anyone help? Cylinder 3 looks like the culprit....
Oh, and the ECU self-diagnosis mode shows up error number 21: Ignition Circuit - "No consecutive ignition signal while the engine is running"