robzilla - 4:01 pm on Oct 18, 2011 (gmt 0)
If this is a Linux box, using the
top command from the shell will tell you which processes are hogging your memory (press the < or > on your keyboard until the list is sorted by %MEM). First and foremost, look carefully for processes you don't need, and disable those. Then monitor the process list for a while to see if there are any processes that take up a disproportionate amount of memory. Also check your cron tab for any jobs that are non-essential.
If you tell us a bit more about the server and website environment, we might be able to look into this in more detail.