If it's doing it with different ripping programs, it may be hardware related. But, sometimes MTR goes crazy on me. Trashing its preferences file works for me. It's titled "com.Geezerbuttz.mtr.plist" and is located:

~/Library/Preferences/com.Geezerbuttz.mtr.plist

~ is your home directory. Delete that plist file and MTR will start up as if it were the first time. It might help and it won't hurt anything.

You might also try an external usb/firewire DVD drive and see if that can work. If it does then that might point to internal drive problems.