i have been dealing with the same problem for a long time. But I found a fix!!! (at least for me)
Both, however, require you to open your xbox dvd drive which will void any warranties you have.
you need to:
1. clean dvd lens carefully with q-tip and rubbing alcohol.
2. lube rail that dvd laser slides on, and the bronze track that the motor turns.
(i used gun oil but any good lube will work. DO NOT USE WD-40!!! or any spray like it)
This worked for me, if you try it I hope it works.