I wonder if the power supply is at the outer edge of its ability with that G3 card in? I had a similar problem, and if I just restarted the machine for a second time in a row, while the hard drive was still spinning, it would come right up. I concluded that the extra power that the hard drive takes as it spins up from a cold stop was likely pushing the power supply past its limits. The second time, with the drive already spinning. all was well. Eventually, I replaced the two drives I had in the machine with one larger and more modern one (much less power usage) and I haven't had the problem since.