Check RM for BIOS firmware updates, I know some of our older machine had some updates.

What have you been using to test the memory? I am a huge fan of Memtest86+ - Advanced Memory Diagnostic Tool but if I have a machine I am not sure about I run the Windows 7 memory test as well.

Might be worth getting a can of compressed air and blowing out the memory slots and reseating the memory again, I have found older machines much more fussy about needing a clean connection with the ram.