Ignition timing - initial setup problems.

[misterforager]

I am really close to firing up an installation of an 'early' AE86 4AGE (1985). All of the wiring checks out ok and there are no diagnostic codes from the ECU. Problem is that I can't get a stable ignition position when cranking the engine (no fuel in the system yet - to be safe). What happens is this:

1 - Crank engine over and timing light shows spark at (say) 5 degrees retarded and remains there. Switch off.
2 - Try again and spark is occurring at 25 degrees advanced - and I haven't changed anything!
3 - Subsequent retries will lead to it being -5, +25 or completely off the scale of the timing light (on the other side of the crank pulley).
4 - Once it has established a position (while cranking) it does 'lock on' to it i.e. it's not scattering around.

Notes:
Valve timing is spot on.
Coil lead is welll away from ECU signals (not an RFI problem).
Distributor signals are fully shielded
ECU is unknown insofar as I've never run an engine with it.
Same goes for distributor and coil/ignitor.
Coil mesurements (resistances) are ok
Dizzy pickup coils ok and gaps to 'teeth' on RPM and spark wheels are within spec.
All earths check out good.
ECU part = 89661-12060
Dizzy = 16040 (last 5 digits)
Ignitor = 12270 (last 5 digits)

I dare not put fuel in and try starting the car fuelled up until I have a stable cranking spark position. Next step will be to connect 'scopes and Logic analyser so as to measure relationship between dizzy pulses on signals Ne & G with respect to ECU output IGt (I believe that IGf is the rev limiter?). I'll stop here as somebody out there may have an answer already. My suspicion is with the dizzy or the ignitor...

Anybody seen this problem before and have a suggested solution?

Many thanks in advance.