You can test whether it is the memoru by using a utility such as Memtest86 (
http://www.memtest86.com). You create a bootable floppy disk using MemTest86, then reboot your PC from the floppy disk, it runs some memory-intensive tests, and displays whether the memory performed reliably or not.
As markymoan suggested, you might want to remove them and try testing them individually, to determine which (if any) are faulty.