G5 Mac must be turned off for several hours to boot successfully.

I'm having trouble getting my G5 Mac desktop to boot. The unit must be turned off for several hours to get it to successfully boot. If not turned off for many hours, the gray Apple screen will eventually show up (no spinning wheel/loading icon). Does anyone know what causes this/how to fix it? The specs on the machine are as follows:

OSX 10.5.4

Dual 2.3 ghz PowerPC G5

2gb ram