Audio from Audigy 2 on a Dimension 8300 suddenly stops

Greetings.

I am experienceing a problem with an Audigy 2 card in my Dimension 8300 system.  There have been no changes to the hardware since its purchase.  I have kept up with Dell's and Microsoft's updates.  I do not believe the problem correlates to the time of installation of any of those patches.  I am running Windows XP Professional.

The sound card was operating perfectly for many years.  The first hint of difficulty was minor poppnig and crackling, but just in the right channel.  This was happening even when no sound of any kind was being played, and did not respond to the volume controls.  Over a period of several months the popping and crackling began to also occur in the left channel under the same circumstances.  All the tests in the diagnostics passed.  (I am referring to the tests accessed by right-clicking the system tray Volume Control icon and choosing "Diagnostics".)  The Device Manager shows all devices operating normally.  I tried restoring the defaults (using the Creative Speaker Settings applet) but nothing changed.

Now, some months later, a more serious problem has developed.  While playing anything with audio, a CD, mp3 files, a DVD played on the local drive, streaming audio or video over the Internet, the audio will suddenly stop completely.  No pops or clicks, no audio channel, just a slight hiss.  Sometimes, just before this happens, the sound in one channel or the other will become highly distorted.  If I stop and restart the playback in a way which causes the application to reset its connection to the audio driver, normal playback will resume (although with pops and clicks) for some amount of time, and then it will suddenly stop again.  That might take just a few seconds, or it might work for 20 minutes or more.  It seems pretty random.  This happens whether I use headphones or play back through the Dell-supplied Altec Lansing speakers.

If I reboot the system, the pops and clicks begin as soon as the sound drivers are loaded, again, even when no audio playback is occurring.  But, the intensity of the pops and clicks, i.e., how many occur per unit of time, varies widely with each reboot.

Pops and clicks during audio playback are often said to be associated with excessive interrupt latency, but I cannot find any explanation for them happening when no playback is in progress.  I have repeatedly scanned my system for malware and have never found any.  I have looked for rootkits, and found none.  Since the card passes the supplied diagnostics it is not an obvious hardware problem.

Any ideas about the cause, or how I might further troubleshoot this problem?  Thanks for your advice.