So the story goes, M$ wanted the 360 to be thinner. The engineers had to then cut the heat sink under the DVD drive to accommodate the smaller space. As a result, the DVD over heats and eventually fails. Cleaning usually does not work, as the laser is sealed from the back and only the lens is exposed. I have attempted to replace the laser in one of mine and this yielded no difference in the performance of the console.
LLAMA.COM has parts like the laser assembly if you are so inclined. Some game repair shops offer a DVD swap but you should know that the DVD firmware is keyed to firmware on board the main circuit board in the console. This requires one to "read" the encryption keys and then reflash the new drive with those keys.
Hope this helps.