Nope, it's not just you. As Steam stats tell us that more than 76,000 people have already piled into Call of Duty: Black Ops on launch day, some portion of that playerbase is reporting intermittent lag issues following them through multiplayer that don't seem to be caused by elderly hardware. Within the Steam forums , the current theory is that it's a resource issue: CPU load for players experiencing the lag is running at or near 100%. Anecdotally, I'd agree that the three-to-four-second full-stops I'm experiencing online definitely don't resemble a network problem.
Black Ops' multiplayer design director, David Vonderhaar, soft-confirmed that Treyarch is aware of the issue. While we await hard confirmation on a fix, peek within for a video demonstration of what many players are coping with.