Sometimes different memory simply does not like to play well together.
I think the Firestix run at 2.1volts, and the Crucial runs at 2.2?
Set the voltage in your BIOS manually for the memory to 2.2 or 2.22, something like that, right at or slightly over what the higher volted set needs and try that.
I'm running the memory at 2.1, as I've heard that running at 2.2 with this particular motherboard (nvidia 680i) has a tendency to fry the ram.
I was ACTUALLY running it at 1.85 a few months ago. Would that have had anything to do with memory errors?
How does voltage effect memory consistency / accuracy?