What is your power supply? Have you added any new memory lately, my friend had the exact same issue and it was because one of his sticks had gone bad. Thus making his performance quite inconsistent until we discovered the issue.
Its not my system. I run crysis, COD 4/5, Warhammer 2, Company of heros, Supreme Commander- 1680/1050 screen at high resolution and my machine runs smooth on all..
CS:Source sucks, and I can't figure out why.... I updated driver I think it helped a little but still get fps spikes low... Wierd, but I don't really care because I am about done with CS anyway.
I got u man ... i understand .... i only play CS: Source because im loving some Chernobyl maps made for Source ...... im very into the holw Chernobyl disaster and so im very addicted about it ..... but if wasnt for that id probably leave CS for good ........... but the fact thats keeping f''ing with my mind is the thought that maybe my videocard is broken or something .... i just get angry when i enter a server on CS:Source and i get enough fps alone on the map ..... but after someone enters or i add a bot ... the frames keep going down like s..t .... now i know that my junk here is far more capabble of running Source on Max no mather what ..... with my old 9600 GSO it went, so i dont doubt .... but when i see those hell frames .... aaaahhhh the irony !!!!!!!!!!!!!!! X(X(X(X( i tried everything i could .... only left for me to do is to try it on a bigger Power Suply .... my is a little small for the gtx260 ..... (Thermaltake TR2 470W real) ........ maybe il get back to S.T.A.L.K.E.R. series & COD4 Modern Warfare again, as the Chernobyl scene is pretty heavy to be found in these games ..... forget the CS:Source s..it ..... someday maybe it will run smooth again ....... when we are not going to even play it anymore .... cheers mate