I have personally put a multimeter on the front of a battery cable from the boot - under crank the battery was dropping to 10.5 volts. The front of the cable was dropping to about 8.5 volts. Therefore the cable was dropping 2 volts under load. The ecu was an autronic that stipulates voltage must be above 10.5 (or 11 - I cannot remeber) volts. The cable was 35mm square so should have been plenty enough.

Anyway - wiring the ecu directly to the boot solved the problem. I have also seen other cars drop to 8 volts under crank and still crank and i think you will find that mose ecu's want 11 volts or so.