Maybe this bug has been listed before - clock stops displaying correct time

Hello! Maybe this has already appeared in the forums. Imac G5 2megs with leopard 10.5.2 - has a wacom tablet and flip4mac installed - otherwise no other extensions or non-apple hardware/software. At some point - at irregular intervals - my clock no longer advances, and any mouse hovering over the right side of the menubar displays the spinning wheel of "death." When I run activity monitor I see that the 'systemUIserver' is not responding. Any clue what causes it to lock-up? Is this a known bug that may be fixed soon? Thanks