Might it be kicking back? If the ignition is much greater than 10degrees at cranking you're likely to get ignition before the piston is at TDC, try putting it down to 5.
You have checked that the timing is spot on with a timing light haven't you? Wasted spark can fool a lot of timing lights (you know this but I'm just typing everything that comes into my head).
Currently you're not on the tables, until the engine speed gets higher than the cranking threshold the ECU is soley using the cranking settings.