Before we start, I would suggest you to check a professional Xbox 360 repair guide with videos and picture. A good Xbox 360 freezing repair guide will show you everything step by step, which can really help you through this process.
Steps to fix Xbox 360 freezing:
1. Take your case apart. Using a fine tip poker, poke the grayed out holes (they are the latches holding it together) and at the back poke the tiny rectangle holes. Take the faceplate off and move the latch up on the DVD drive up as you remove it. Unscrew the metal case that is just under the plastic case.
2. Get the DVD drive out and unplug the cables.
3. Get the fan off and the motherboard out.
4. Flip the motherboard over, and, using the poker, pry off the X-clamp under the heat sink by popping up the corners. The heat sink is the big silver box with slits in it.
5. Unscrew the heat sink to reveal the CPU and GPU.
6. Clean the GPU and CPU (critical step to stopping Xbox 360 freezing)
7. Apply arctic silver thermal compound (you can find a $1 tube of this at any computer store) generously over the GPU.
8. Now Let it set for 15 minutes.
9. Put 2 nylon washers on each of the screws from the heat sink. Put the screws back in the motherboard. Now, with the screws pushing up through the motherboard, add 2 more nylon washers to the screws.
10. Put the heat sink on top of the screws, pushing them down a little, then screw them up. Make sure they are tight. This is will really help with Xbox 360 freezing.
11. Now put everything back together. Turn it on.
There, you should now have fixed your Xbox 360 freezing.
If you need help with pictures or videos, again, I suggest you check out an Xbox 360 freezing repair guide.
Xbox 360 freezing repair guides are fairly cheap, around $25, and they save you over $100 from sending it into Microsoft. Plus, Xbox 360 freezing repair can take up to 6 weeks if you send it in to Microsoft. 5/16 to mm