It might be an issue with your clock capacitor in the XBOX itself on the Motherboard. You may have to take apart your Xbox, and will have to check the Motherboard for any corrosion leakage from the capacitor.
If there is, you can easily remove the capacitor (or re-solder it if you feel talented enough) and clean off the residue.
Should be working fine after that!
It is to be noted though that you can only do this to XBOX systems from before 2006