Recently while playing two rental DVDs (one a new release, the other purchased used from the rental store) the sound on the new rental went dead about 15 minutes into the movie. It can back about a minute later but kept getting louder, then softer; louder and softer. The other DVD just did the up and down thing. I'm wondering if it's the DVDs, the player, or the TV.

It's an Aiwa (about $85 worth) plugged directly into the back of an aging RCA stereo TV. Never had this problem before, but then I haven't had the player very long. Any ideas?