If you use a high bitrate on your MP3's you may be taxing your CPU too much. Though you have a high end PC, using HL, ESF's particle engine, and the huge maps while FMOD.DLL decodes 300kbps frames of you finest tracks, you could start to beat on it a bit to much. This isn't including all the other stuff you could be running in the background. Try seting the amount of memory windows allocates to HL much higher, along the lines of 500MB out of that Gig you have. Add -heapsize 512000 to HL's launch line after
Like this: C: \sierra\half-life\hl.exe -heapsize 512000