tracks muting

Ever since the most recent upgrade, GarageBand keeps spontaneously muting tracks.

That is to say, if I solo a given track to listen to it...and then turn the solo off and return to my normal track setup...GarageBand will play the composition correctly perhaps a couple of times. Then, seemingly at random, it will start muting tracks. And not necessarily one of the prior tracks I'd been listening to solo.

Just...randomly mutes certain tracks.

I've checked to make certain the tracks are not muted by me, nor are they locked.

It appears to happen in both new projects I'm working on, as well as older projects I open. I've not been able to detect any rhyme or reason to the behavior, except that it eventually occurs after soloing one or more tracks and then returning to the standard playback.

Sometime is mutes the other tracks immediately...and sometime it will play through everything correctly a dozen or so times...before the mysterious muting occurs.

I'm running OSX 10.8.3 on a nearly-new 8-core Mac Pro workstation with plenty of RAM. I've not noticed any other contributing factors; it seems to occur regardless of where the source loop library is stored, and regardless of whether I am working off of the SSD, or one of my RAID drives. I use a variety of Pro apps from both Apple and Adobe, and I'm pretty tech-savvy when it comes to this sort of stuff...and yet I'm left scratching my head.

The only solution seems to be closing and re-opening Garageband. Then everything returns to normal...at least until I solo a track again, then the problem reappears.

I've seen similar posts on this topic here and there, but no workable solutions. Any ideas?