Ah, I used to love that error so much (not really though). It explains itself: you have packet loss. I used to get it a lot on 56k, but I tweaked my modem a little and that helped (and actually I think Steam helped cure that too since I got it on WON a lot). In fact, I hardly ever get "Warning: Connection Problem" anymore. To get rid of it, you'll have to tweak your connection (rates for HL and for your actual internet connection) or your ISP is screwing around with some stuff for a bit.