Echo/static with Polycom 301

I’ve just set up AAH. I’m running a dell 2.8ht, 512ram. SIP from Polycom 301 to *, then IAX2 to Junction Networks over a cable modem. The local network is a home setup with a linksys router, but with 4569 set to high QOS. I hear a medium amout of of static and echo on the SIP side (not consistently - there will be 10-20 seconds of clear sound, and then 10-20 seconds of stormy weather), but the other end reports a very clear call. I’ve installed g729 licenses on the * box. I’ve tried setting the 301 to uLAW and to (I think) g729. Any thoughts on what might be causing the quality problems on the local side? Any suggestions for logging or diagnostic tools?