session_start() may take 5 seconds :(

My web-app sometimes takes just over 5 seconds to execute a single start_session() statement. No other connections are open to the site when i hit the button that makes this happen, the session is just 78kb on a local disk, and it's consistent behavior. So far, it only happens when i request certain page content via AJAX, but I'd like to get rid of it asap. Strangely, some other page content requested under the same conditions via the same pipeline returns without the delay. It doesn't do anything differently except the business code, and the session is opened in app-wide generic code that is executed before the business code is executed. The very timing with microtime() happens before the business code executes. I've already googled, but haven't found anything useful. It's on a windows box (latest wampserver) and due to a ubuntu firefox limitation i can't test on linux at the moment, nor run strace on it :(