Are you on X3E and using 64 bit everywhere?
(and +1 Wookiee).
Here's what I recommend first:
1) Make sure you are NOT using the Windows firewire legacy driver. That was for old versions of mix control.
2) Uninstall mix control (whether you have 3.4 installed or not) and reboot.
3) Install mix control 3.4
here and Reboot.
4) In Mix Control go to File -> Restore Factory Defaults
5) In Mix Control go to File -> Clear All Settings.
6) Try it.
Any further issues you are probably looking at latency troubleshooting next...