Is this only occurring when you're using your Xbox 360? If so, I suggest trying a different display cable on your console. If the same issue occurs then too, it sounds like your console may be overheating.
In this case, I suggest making sure your console has plenty of room to 'breathe,' and that none of the ports are blocked up with dust at all. If the ports are blocked, you can try clearing them out with an air canister, or a vacuum hose.
If none of these steps resolve your issue, then you'll want to set up a repair. You can check your warranty status and set one up
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