Source does not run like a dream. I've had some of the same dreaded sound glitches I've had in HL2 in Bloodlines.
When it slows down like that, tell him to exit the game and restart, then get back into it. The code suffers from some pretty bad memory leaks, that get worse the longer you play. I've never had any problems before about six hours or so, but then again, I exit pretty much any unneccessary processes before playing.
My rig isn't as good as yours either, I need to upgrade, but have been trying to wait until all the new standards come out.
And I have money.