You have to edit the afterburner.cfg file to overclock/underclock, Afterburner isn't retaining the downclock overclock info because you need to enable unnoficial overclocking. Here are the directions how:
http://forums.guru3d.com/showthread.php?t=338906If you're on Windows and it's causing system crashes you may also need to edit your registry by setting "EnableULPS" to 1.