Discs won't play

I recently rented several DVDs of Alias at the local video store. I was able to watch a couple of them, but then all of the sudden, they would no longer play. I tried several times and it would just attempt to read them with nothing actually happening and then it would either freeze up or spit the DVD back out. I thought it must be the discs, so I took them back and rented the the same ones from a different store and the same thing is happening. All other DVDs we try to play are working fine, so it doesn't seem to make any sense. Can you help me figure out what is going on?!