Is there an extension/INIT that loads at that time? Possibly in there due to maybe someone installed an upgrade that if it's not presently there, it might force it to reset and load without the extension/INIT plug in.

I know some G3 upgrades for the PCI series powermacs did that. It would start to load, then reboot (and it installed the firmware into the PRAM to support the upgrade) then it would reboot. Try booting off a floppy/CD and see what it does. It might be something loading. Also, check the power supply. It might pull too much power when the hard drive spools up and the power supply cuts short causing it to restart. Remember, the startup of any machine is usually the worst load ever put on the power supply. This is because so many things (floppies/HDD/components) starting up and surging the power at the very instant they start powering on, and they usually do it all together at once.

So either try booting from a CD, or get out your multimeter and start testing the DC out from the powersupply and when it restarts, see if there are any anomalies right before it reboots.