Step 1: Unplug device.[br]
Step 2: Vacuum every part of the power supply.[br]
Step 3: Use an air can and spray every inch of it.[br]
Step 4: Vacuum again to remove loosen dirt.[br]
Step 5: make sure psu is clear, and laying flat.[br]
Step 6: Lay psu at a different angle for a short while to help free up any loose dirt.
I have tried doing this many times on my Xbox 360 when it was doing this, and it has stopped it eventually if not instantly. Hope this helps.