PCI-E frequency problem...

[mcwildcard]

I've got my GA-945P-S3 running an E6300 @ 2.45GHz, I've run Orthos for a couple of hours with no problems at all, but one thing is puzzling me.
The FSB is running at 350, but if I set my PCI-E frequency to 100 it defaults back to stock when I try to boot the system, however if I leave the PCI-E setting to Auto, it's fine? This only happens when I go over 300 on the FSB.
What does this mean?

 

[mcwildcard]

Well if it doesn't ring any alarm bells then I'll see how far I can push the FSB, I'm getting a GA-965P-DS3 soon anyway, but there's no harm in seeing what the 945P-S can handle in the meantime