Possible new headphones out of phase

Hi,
I recently picked up a new a pair of cans for my audio needs: Audio Technica ATH-M50. They have an extremely accurate sound and I am pretty satisfied. I have a feeling that the two sides might be a little out of phase because at some points the sound is extremely clear and others its a little muddy and seems covered up. It comes in cycles which adds to my thinking. They are literally brand new and I haven't even had them a week and I noticed it right away. I have a feeling I might be a little crazy but it still seems like a valid explanation. Does this sound like an issue? If so, they have a year limited warranty and I could easily take them back to the shop even. I'd like someones opinion on this if possible.

Thanks

With what media/file types?

Why would being out of phase be a "sometimes" thing?

Some files types have phase anomalies.

If the L and R headphone grounds were to remain tied together but become disconnected from the plug's ground (or the plug's ground from the jack ground - or the jack ground from the headphone amp's ground), everything that's panned to the center would disappear.