Xbox Crashes- Then Ffxi Disc Says It Is Unreadable..

So, a few months ago, my husbands xbox broke. We went out, got a new one. Recovered gamer tag, then did the 6 hour download. Got all his player info onto the nex xbox, and its all fine, he starts playing. As he is logging out for the night, the xbox freezes, so we just turn it off. Next morning, go to play the game, and its like the disc is unreadable. We spent hours fiddling around, and checking websites for answers. Anyway, my husband decided to swap our hard drives over, and when he switched it back it worked. So i was like, ok, you got a dodgy xbox, we can live with that.
Anyway, yesterday my xbox broke, so went out got a new one etc etc. The exact same thing happened with my xbox. Also, it takes so much longer than normal to get to the PoL viewer. It sits for ages on the blue screen, before getting to the login information.
I was wondering if anybody knew of anyway i can fix this problem, as its annoying when we have to log out mid party to get the other back on line, coz the system froze.
And before anyone says it, no we dont wanna switch to PC or ps3 lol

my xbox is old.... i smack it... sseems to work everytime

Try defragmenting your hard drive. If you don't know how this is done, turn on your xbox. On the Xbox 360 dashboard, go to the "My xbox", then go to the last slot. "System Settings". From there, move to the memory tab and press A to open it. You should see "Hard Drive". Now, when "Hard Drive" is highlighted, press the Y button. It should now say "Rename" and "Format". Highlight either one of these options and press X X LB RB X X. A blue text screen will pop up asking if you want to perform Maintenance. Press "Yes". This frees up extra space on your HDD by deleting "cookies" and checking for corrupted files. Hopefully it helps.