<sigh> Yes - I probably know why. You're most likely running out of space - before you go "But I've got like 5GB left!?!?", let me explain...
The iPod Touch has two partitions - the first partition contains the applications and the operating system and is FIXED at 300MB. The second partition contains your media (music, video, settings, etc.). So, what's happening most likely is that you're running into the 300MB barrier. What you need to do is move some of the stuff off the first partition onto the second partition and put a "symlink" in place (like a pointer or shortcut or alias). If you have MobileTerminal or Term-vt100 installed (or you can send a command via SSH), you can verify the space available by typing "df -h" at the command line.
Here's how you do it the easy way. Uninstall a couple applications. Then in Installer, add the following source - sleepers.net/iphonerepo - make sure you spell it correctly. Then install the BSD Subsystem (if you don't have it installed already) and the BossTool. Run the BossTool. At the very top you'll see an option to free memory. It'll tell you how much you have and what you can move to free it up (wallpaper, applications, etc.).
Now just a warning - this is a low-level patch so things can go wrong and you may have to jailbreak again. But it works a good 90+% of the time... And it's the easiest fix unless you want to type in a bunch of commands by hand...
Pass it on...