Afraid the problem is your dvd player mate. The new releases of the Alien films use Seamless Branching which is having some strange results on some dvd players, some can play them 100% and others skip/restart/freeze.
Only way to solve this is to get a new dvd player, out of the 4 in my house only my xbox will run them perfectly and then it froze up on one of the extras discs once but the problem is getting a player which runs them without problems.
I did voice my concerns a while before the set was released about the seamless branching issue and no one seemed interested, now people are reporting problems and blaming the sets for being faulty when there is nothing wrong with them.
Think it would be a good idea to create a list of players which can play all 4 films perfectly and a list which has trouble playng them, what do you think?