i have the same problem with my g3 imac (well, i did have the problem, now i just don't put it to sleep). i believe that the problem is with the maxtor disk (i think apple actually had a recall on some of the drives). sometimes mine takes a few seconds extra to start up after i push the power button and sometimes it will beep on startup. when mine would beep on waking up, it would just continue beeping and the disk would never turn on. never putting the computer to sleep seems to have worked for me, but i think replacing the hard drive would solve the problem all together.