Nothing else has changed in MC Center or Remote. My music library is held on my Mac Book and I stream wirelessly to a CA Stream Magic renderer.
The wireless is the most suspect thing you've identified in your description. Wireless can be very unreliable and inconsistent. It can change day to day or hour to hour depending upon a lot of conditions. Just having a new device of some sort in your house can change wireless performance (due to interference).
Adding music to your library should not have affected the ability to play other music. It's probably a coincidence. It's also normal for wireless links to play lower resolution files, but drop out with higher resolution, as they require more bandwidth. So on a questionable link, the problem will show up on the bigger (higher resolution) files.
This *might* be a CPU issue, but your whole Mac would seem to be running a bit slowly if that were the case.
I'd recommend moving the Mac VERY close to the renderer and trying again, as a test. If you can do it, running a real network cable would eliminate wireless as part of the problem.
Brian.