Well after trying all the common 'ICS' 4.0.x mods for my SGS I9000, I think the hardware is simply not up to the task.
Ecstatic now that I've gone back to 2.3.6 - Darky's ROM. Version 10.2 is nutted right down now and fully optimized to get the most out of your phone. Using 'GO launcher' for the UI which is the best. Gonna stick with this now after fiddling around with different ROMs forso long. It should serve it's purpose another year or so, so maybe hold back on the SGS III and see what comes out this year.