|Do you have a slim or one of the old ones made before late summer 2008 .|
The GPU could be getting hot or it could be getting loose from the motherboard as it is attached using the BGA (Ball Grid Array) which uses solder balls to attach the CPU and GPU to the mobo, overtime hair line fractures can occur which can 'cut' the link between a single contact on the GPU and the mobo, if this is the case you can do a few things to fix it.
If you haven't opened the xbox and it is still in warranty send it back to microsoft.
You could try to fix it you self by trying to reball it in an oven- which i don't really recommend unless you know what you are doing- you can google how to reball an xbox 360 in an oven.
Another problem is a bad composite/HDMI cable which can be replaced.