replace your cmos batterie it sounds like its on the verge of dying.

It certainly sounds like the CMOS battery is dying!

Without a battery in good condition, your system configuration data is not being stored, and it is needing to run setup to detect the equipment installed in the PC. Running for a period of time will charge the battery somewhat, ensuring that configuration data is retained for a while. this explains why it starts up fine sometimes.

But left turned off for a period, the battery will be going flat again, and the dta will be lost. replacing the battery is a simple and inexpensive process on most systems.