that the pickup may be dirty? Try buying a dvd cleaning disc. Maxell makes a nice one. I used it on my PS2 and it solved the problem I was having with the movie freezing or giving me error messages.
I recently gave a friend a gift of a Toshiba 4990. In the few times he's used it, it stops suddenly and he has to re-start the program.
Any ideas on why it does this?