I haven't used and didn't know about the diagnostic tool but you can test RAM with MemTest86
- you can download the iso from their site or get any Linux CD, where it can also be found and run.
With system behaving as you describe, memtest should pick up the errors immediately. If you do get errors, you can try removing the memory module and cleaning the contacts and trying the test again. Otherwise, contact Fujistu (if it's still under warranty) or get new memory.