I had this problem as well, however it was most definitely due to very poor internet connection for gaming. What I think was happening is that my school ISP was throttling my upload, but not download, to the server. Make sure you can see your lagometer and frame rate, so that when this happens you can know roughly what kind of trouble you're having. If it was a problem with your computer, then your framerate would drop, if its an internet problem you'll either see yellow at the top of your lagometer, or at the bottom. Top is missing packets, bottom is the packets taking too long. If neither of these happen, it could be a problem with tremulous, or with your tremulous configuration (/exec default.cfg, and reset the settings you really need), but I think its an internet problem.
