Blu-ray player help [BDP-S780]

I have a question that maybe someone is able to answer. My sister has a Sony Blu-ray player, model # BDP-S780, hooked up to an HDTV, via an HDMI cable playing back at 1080P. Whenever we put in a Blu-ray disc, and played the movie, the audio was out of sync with the video. Sometimes it catches up, sometimes it doesn't. My question is Why? Thank you for you time.

Just an edit to my original post. I think the Blu-ray player is hooked up through an AV reciever, I don't know if that has any bearing on the issue.

I would definitely remove the AV receiver out of the equation, it might be applying some sort of processing to the signal prior to handing it off to the tv. Other then that, it could be an incompatibility between the tv and the bluray player that needs to be addressed via firmware update.

So both audio and video are going through HDMI via the AV receiver?

Or do you have a separate digital cable for the 5.1 audio?

I believe the audio is through the HDMI cable. Is there a work around for this? Thanks again.

You can try getting an optical cable to go from your blu-ray video to your AV receiver. You'll still get 5.1 surround and the HDMI will still display your monitor's max resolution.

I'm not sure if that would solve your problem, but it might help if you separate the two.