Problems with CS:S

Ping would definately cause lag. Fps would definately cause lag. Choke and loss could both simulate lag. People on server/screen effect all of those stats.

I doubt it's spyware because he said it only effects certain maps.

Also, check your cl_cmdrate and cl_updaterate. They should be around 30, but you can tweak them to some better effect hopefully. If you've got a lot of network stress, leave them low. If you're dedicated, you can put them a little higher. On a wired network, I run around 70 for each.