Sites go down, go back up once I login via ssh

[Greg Walker]

This is the weirdest thing that has ever happened. I have a new server with plesk running on centos 5.x. After 24-48 hours all of our sites appear to go down... can't access via browser. Then all I have to do is login via ssh and then voila... they are back up and accessable again via browser. What could be causing this???

 

[IgorG]

Apache logs should be investigated at least...