Have you tried the mmcs regedit tweak yet?
Leave the multimedia class scheduler service running, but disable its network throttling "feature":
- Open regedit
- Go to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Multimedia\SystemProfile
- There will be an entry for network performance throttling, default value is 10; set it to FFFFFFFF hex (willl look like 0xFFFFFFFF to the right of the entry)
I use this on my 64-bit gaming rig. It's worked wonders for me; my pings are super stable right now. I tried helping another guy with CS:S who was having the same issue... tried duplicating the problem on my wife's rig, 32-bit, mmcs enabled, but play was just fine on her machine. Not really sure what else I can offer.