See if it acts up on a known good disc. If it does, then you know that it is time to kick this player to the curb (or at least the closet) and buy a new one that works better.
If it doesn't act up with known good discs, then it is probably the quality of the discs you're getting from the rental place. Believe it or not, a very very thin coat of good car wax (buffed in to complete clarity) can actually rescue these discs.
Best of luck to you.
Gary
PS. I think I've given you good advice, taking all factors into account. If you agree, please accept this answer below.