If the chip does the speed at lower fsb and the mem is good at 200 asynchronous at whatever timings, it's almost certainly the northbridge that's holding it back (no need to bother fiddling with the mem timings), you need to up the chipset voltage to however high it will go, if that doesn't work then i'm afraid it's probably a volt mod and some extra cooling on the northbridge, so you might just stick with what you've got.