Well DarkFang there are a few things you should know about CS:S at the moment.
Steam has put the game through several updates and now its engine is much more similar to TF2 (not sure if your familiar with it) but in other words it requires a significant amount more from your overall cpu usage so many users may not be seeing their previous sky rocketing fps (like I used to).
Personally my FPS has gone from a 60 to 30 and this is after tweaking my autorun scripts to optimize my graphics. At this point being that CS:S now requires a higher min. to run as smooth, I would recommend you check fpsbanana for an autoexec script which basically configures shaders and other environment elements.
Another thing you can try is to adjust your rates. This setting is usually included in most autoexec's for CS:S but you can also manually enter in console cl_cmdrate 101 cl_updaterate 101 and rate 25000 (or 30000 whichever your performance is smoother with). This helps your client side net upload and receive packets from the server better and smooths out gameplay and best of all helps your shots register more accurately.
The last bit of information I can give you is to possibly change what server your playing on and see if maybe its server side lag which is causing you to lag. As a result of the updates many servers still haven't gotten a chance to update their servers to run with optimal performance (this applies to maps as well especially a number of ze_escape maps which are broken now from the update). So there isn't much to be done if this is the case and your server is at fault and not your rig. (This is something I've faced personally so far in a few servers). Hope I was of some help