Vintage Mac G4 Cube

Attn Vintage Mac Enthusiasts...

One of my hobby macs is acting up. It works almost perfectly except after being on for about an hour my menubar will disappear, except for the right side where the icons are. A quick click anywhere on the desktop brings it back.

What could be causing this erratic behavior?

G4 450 1.5G RAM 160G IDE HD (30G free) OSX 10.4.11 OS 9 Classic 23 in Cinema display ADC

Footnotes:

Screensaver is not 3rd party

Energy saver settings are 'never' for dimming screen, engaging screensaver or putting the HD to sleep

OS 9 Classic enabled at startup

DU and Diskwarrior run often

HD is Apple formatted - 1 partition

Conflict Catcher 9 enabled