If you have Root Explorer (or any root file explorer) installed, you will need to open and scroll down to /system. In here, see if you have "recovery_from_boot.p" there - if you do, you will need to change your Mount R/W to Mount R/O and rename this file to "recovery_from_boot.p.bak" - then open up ROM manager, reflash clockwork and then you should be able to reboot into recovery with no issues.