first thing to try is connecting the monitor to the video output on the motherboard and not the graphics card. it's possible the bios reset itself to the onboard and not the pcie slot.
also replace the battery on the motherboard. that's a very old system and that battery may have died which will cause the bios to keep resetting itself. if it works with the onboard video and not the card, then check the bios and set it back to using the pcie slot after the battery is changed.
that's where i would start if i was working on it.