It's the 256MB RAM, happens on my 3GS daily. I have a nifty app called System Activity Monitor that clears RAM when you enter it (sadly it's not for free). The phone runs great until there's like 2-3MB free, then it needs to clear stuff before it can open anything new and during the clearing it's slowing down and the response is jerky. It eve manged to crash the springboard a few times (opening an app caused a hang and a soft reboot after ~30s).
If you don't want to pay for the app, you may want to JB and get SBSettings, there's RAM clearing on command built into it.