This has happened to me as well, thought I had been banned for some reason, last time it happened I E mailed and asked about it, was told site just gets busy, not a good explanation, still convinced I was banned
seems to be pretty common, also, do you like me seem to wait an absolute age sometimes to navigate the site, submit and wait, and wait .......i must say though, it is usually worth the wait << (er' is this a crawl?)