It has nothing to do with tweaking, people have reported the same. Sending boards back, getting RMA replacements which can't do the same settings as the board they sent back. It's completely random.

Running sync (which was faster) took so much voltage, thats why it took me 1.7 pretty much to get that sort of FSB out of it.

ICFX was a nice tweaking board, but RD600 is the worst chipset out of 965/975/680i/P35. Slowest clock for clock because of the dual bus design to give full unlinkable memory, poor raid performance, seemed very fragile with the amount of dead boards.