Take the VRM (DC-DC board) out and clean the contacts. While it's out, look for evidence of overheating. If anything looks scorched, you probably should replace it. Check the rest of the chassis & heat sinks for dust, etc. since Cubes are marginally cooled unless you add a fan. If you've upgraded your cube, check that the fan is still working. Unplug the DVD drive's power connector and see if it's failed and overloading the 5V & 12V supplies. It seems unlikely that 3 drives would fail so quickly.

Another possibility is that the power supply brick is failing. These are getting old enough that some of them are getting flaky. It should be putting out a steady 28V DC.

Check the disk drive topics on www.cubeowner.com, which is THE place for cube and early G4 info.