Error while trying to synchronize audio and MIDI in GarageBand (Not using MIDI!)

I get this error message if I try to use any other programs while playing back two tracks in GarageBand. It seems to be related to the level of activity in the other programs (namely, Safari)--if it's trying to load a lot of images or I'm scrolling through a lot of content, the playback gets distorted and this error message pops up.

This is audio recorded from Skype using Call Recorder. The microphone is a Blue Snowball. I do not have any MIDI devices.

This didn't used to happen, so I'm not sure what changed. It seems strange that the 3.1 GHz processor and12 GB of RAM on my iMac can't handle playing back audio and browsing the web simulatenously, but maybe that is the case. The audio files are uncompressed, so perhaps that has something to do with it.

I realize there are a lot of other threads about this, but most have to do with Logic and/or they're recording directly in GarageBand.