My Quadra 700 does the exact same thing when booting 7.6.1. (Maybe other OSs but I haven't run any others in a long while.) It chimes, starts just enough to turn the CRT LED green, then it reboots. I've been told this is because 7.6.1 needs something at boot that macs as old as the q700 don't have, so it has to load the file or whatever into PRAM (I think) then reset and boot normally.

Apparently I even made a video of it a little over a year ago. http://www.youtube.com/watch?v=Jb-JKOSv7gk