hey i was having the same problem with my garageband playback, if airport is on, i get really choppy playback, but its perfect once airport is off. i did some research found two suggestions that may help your situation. the first one solved my problem all together.
1. reset disk permissions, use Disk Utility and fix the disk permissions on your drives, software updates and other things can get these out of whack and cause sluggish performance or application failure, the only thing to note about this is that sometimes it wont fix third party applications, but it fixed my problem with garageband in under 10 minutes
2. revert to an older version of the Airport software, i didnt look into this, but some people said they originally didnt have a problem with Garageband until they performed a software update, so reverting to a previous version of the Airport software may solve the problem
my best guess is that the problem lies between both of these solutions. the newest Airport updates may have screwed up the disk permissions on my machine, so Fixing the disk permissions got everything back into line and everything runs good now. another thing that affected the playback was the Buffer Setting under the "Audio/MIDI" tab in the Garageband preferences. if it's set for Large Buffer Size (Maximum number of simultaneous tracks) my machine performed smoother MIDI playback, but if you're working with real instruments you may need to leave it on Small Buffer Size (Minimum delay when playing instruments live). hope this helps get things running smoothly!