It could be a hardware problem (try switching your RAM modules in and out to see if one of them is screwing you up), but those error messages are common even without a hardware problem. Try assigning more memory to your applications from the Get Info panel. Also, if Virtual Memory is on, make sure it's set to only one MB greater than your physical RAM (i.e. if you have 448 MB of RAM, set VM to 449 MB). Or try turning VM off altogether and see what happens. If those don't help, I would bet that you have a software conflict of some kind.