Intermittent stalling during audio/video playback

This issue is driving me up the wall. I hope someone can help.

Often, while listening to (for example) iTunes, playback stalls and there is silence for several seconds. If I listen to music for an hour, I guess I might experience the issue anywhere from 3-6 (?) times — though sometimes I can go for more than an hour without it happening. But: it doesn't just happen with iTunes — the same thing happens from time to time when using VLC for video playback (usually accompanied by some video artifacts), or HTML5 video playback within Safari. (I don't have Flash Player installed, though I occasionally use Google Chrome for Flash content; if I recall correctly I think the same thing sometimes happens with Flash videos/audio within Chrome too.)

Along the same lines, now and then I notice that when attempting an action within Mail.app, a task will appear in the Activity Window that's something like, 'waiting for the hard drive,' with a barber pole and stop sign. This will delay my action for several seconds. I suspect this is a different manifestation of the same underlying problem that causes the playback stalling.

I DON'T typically get the swirling beach ball of doom when any of these things happen. Mouse pointer does NOT freeze, and the system seems responsive in all other ways during these episodes.

I'm pretty Mac savvy, and I make sure to do regular hard drive maintenance (verifying volume with Disk Utility, rebuilding directory with DiskWarrior, etc.). My startup partition is only about 2/3 full. This week I used iDefrag to defrag the hard drive, thinking it might fix the problem, but nope it still happens. SMART diagnostics have never shown a problem.

When researching this issue on the web, some folks have suggested keeping an eye on the log files (Console.app), and Activity Monitor. I definitely haven't found anything in the log files that coincides with incidents of stalling. And nothing in particular seems to 'spike' in CPU activity in the Activity Monitor at the same time as the stalling; at least not that I've noticed.

I'm pretty good about keeping apps up to date, and I don't have too many apps running in the background. But the problem persists even when I've quit everything else except the app that's doing the audio/video playback.

This has been going on for a long time, and nothing I've tried has helped. Any advice or suggestions, anyone??

- Gord