I think it may be your processor. Games run on the Source engine require a solid CPU to play games smoothly. (Not saying my computer is very powerfull) but even I lag when there is more than 20 people in a server. Anything lower than that and it's very nice. Try creating a server by yourself with no bots and see how smooth it plays. It should run quite nice, but then again you said you got about 33 fps on the stress test. Hmm....