The laser is most likely the cause of your issues.
So you have three options.
1> Tweak the laser to make it stronger, it will eventually die but this is the cheapest, fastest fix. [http://xbox-experts.com/tutorial/how-to-tweak-the-dvddrive-laser/downloads/XBOX_360_POT_CALIBRATION_V1.0_by_Moda.pdf|PDF Link] (Difficulty: Hard)
2> Replace the laser. (Difficulty: Low to Medium)
3> Replace the DVD drive. (Difficulty: Physically easy. The software can be hard to very hard) To replace the whole drive you need to extract the key from the dead drive and spoof it onto the new drive.
All of these will void your warranty.