Before calling your ISP.... are you getting bad lag spikes on any other internet games? If so, then yes call or the seeding could be whats going on here.

However this sounds similar to me. I know when they first updated CSS I couldn't play on server to save my life, the servers ping would be fine, I'd join and 30secs later I'd spike like crazy. I know some people did the crtl-alt-dlt thing while in game and clicked on -all user- and then there was some sort of svhost service having to do with the audio that was messing people up. Or people simply closed the steam application from running in the background. However none of these worked for me. The only thing that worked was a series of steam updates.

So you need to narrow it down, is it everything spiking or just CS? If it's just CS then your ISP can't do anything for you.