Speaker cut off at low volumes

I have two ceiling speakers connected to a Pioneer VSX-816 receiver. When watching movies, one of the speakers is cut off out of the blue. Here's what I could notice:
- it happens 1 or 2 times per movie
- it only happens when the audio in the movie is very low
- it happens randomly to either speaker
- the speaker comes back if I turn the volume louder
- other times the speaker comes back after I power off/on the receiver
- I switched the speakers to the second zone of the receiver and the problem is the same
- the first time I connected the speakers, I had to turn up the volume in order to "activate" the speakers. They were fine after turning the volume back down.

What could it be? It seems weird.

The receiver is probably defective, but what makes it behave like this?

After some more experimentation, I could hear the fan starting and stopping repeatedly when one of the speakers is cut off. So could it be that the fan is defective? I'll try to buy a similar one and see if that fixes the problem.

The amp is about 6 years old.

Why ceiling speakers? This is the bedroom, and the speakers were already installed in the house. I don't know what brand or impedance the speakers are. They don't look special so I guess they should be 8-ohm. I'll ask the prev owner though.

I have a newer Pioneer receiver that drives just fine the ceiling speakers from the family room.

For now, the old amp goes to the dump. I'll get a NuForce Dia mini amp and connect all the devices through the TV (which has a optical audio out).

No, it's not a whole home setup. The family room and master bedroom are wired with ceiling speakers. The speakers are accessible in their respective rooms.

I lost my patience and the receiver is already in the garbage. I just didn't want to bother assembling it back after pulling out the fan and not finding a matching one at Fry's. We'll know when I connect the new mini amp. It would be quite embarrassing to still have the issue :-)

I just want to confirm that everything's fine after replacing the amp. It was interesting how just one of the speakers would be cut off. This made it less obvious that it's an amp problem.