The problems you describe are typical of shortage of memory (graphics don't display properly, messages about resource shortages, etc.)
Assuming a) youv'e got lost of RAM, and b) none of it has gone faulty and is being blacked, and c) you have got a large enough swap file, then you probably have a program with a "memory leak"
Identifying these can be a bit of a b*gger, but there are some well known culprits, like Microsoft Outlook crashing, or Micrsoft Access.
Is this any help?