There seem to be a lot of people with stuttering issues. It's all over many boards. Someone in halflife2.net posted this from someone at Valve:
Set snd_async_fullyasync 1
Reset snd_async_fullyasync 0 to turn it back off.
This may cause issues with voice synchronization, but should also reduce the stuttering.
We're looking into the problem, but it would help to know if this
solves it for you.
Rick (Rick Ellis, Valve Software, Senior Engineering Lead)
If you're still getting sound stuttering, or popping, or static after the above (which should help with HD pauses, but not the sound
pauses), then change the following setting in config.cfg - eliminated
sound stuttering completely for me.
snd_mixahead is originally set to "0.1"
Change it to a higher number - the number will vary by PC, but for me it was:
That'll fix it - if it doesn't, set the number higher. If you set it lower than the default setting, the popping/stuttering gets much worse.
I'm at work and only saw these suggestions this morning, so i can't confirm that it works, but good luck.