That happens to me occasionally. I never know when it'll give me troubles. I can go for weeks with no problem, then zap! it screws up again. Sometimes it helps to cancel the subscription and resubscribe (I do this through iTunes, not from the boards). Sometimes it takes a reboot of my computer...although I suspect that may not be what's helping, but it does seem to help occasionally. I'm computer ignorant, but sometimes it seems there's a virus or something blocking certain links, and a reboot will clear it.
Like rotlex says, it's the DIS Unplugged that gives me the most trouble...although I have had it happen once or twice to other podcasts.