Intermittent slow response on SuperMicro SuperServer X8DTT running RHEL 6.1 64bit

We have about a dozen SuperMicro SuperServer model X8DTT running RHEL 6.1 64bit. The exact kernel version from uname is  2.6.32-131.0.15.el6.x86_64. We've noticed many occurrences when a server becomes unresponsive for a period of time. Open ssh/terminal windows that were working just fine suddenly slow down and exhibit response times (i.e., time to echo typed characters) measured in tens of seconds. The machines are not heavily loaded, are not swapping, are not filling a log with disk error messages, etc.  I've never found any indication at all in /var/log/messages.  A reboot fixes this symptom in my experience; alternately I just wait 5-10 minutes and it goes away. Telllingly, the problem has occurred on a server that was previously running RedHat 5.5, and while running that version it had no such problem.  So I am leaning towards a hypothesis of a RedHat 6.1 kernel problem on this particular motherboard.