There are a million videos on how to repair the 360 out there on YT and the net and here is one more to add to the pile. In this video the biggest emphasis on how to repair what the title says basically comes down to the TIGHTNESS OF THE SCREWS, many people don’t realize that due to improper tighness on the gpu and cpu heatsink screws can lead to all of the stated above problems. Recommended that you go down to your local hardware store and find some 5×10 fine thread philips screws and some small washers that will fit over the screws, go to your local pc store and pick up some thermal compound and give this a shot. Total cost shouldn’t run you anymore than . Once you get everything put back together, listen for the fan speed. If it sounds like it’s racing than you’ve probably got the screws too tight or not tight enough. If you get 2, 3, or 4 rings, once again, re check the tightness of the screws and try again. Finally, if you still don’t have video or the video is good and then goes bad again…..Check The Tighness Of The Screws 
Hope this helps a little bit.