If your G5 is the liquid cooled PPC type these instructions are on the money. The power supply needs to be tipped/angled in about 45* before it will clear the case and pull free. There’s not enough room in the case to do this easily. It scrapes hard on the case and it takes some force. Mind the posts that align the processor to the motherboard. Do not bend them when removing the power module. Move the thick bundled wires to make as much free room as you can before you muscle it. It really does have to be forced out. My G5 was cutting out in the middle of use, then coming back on a few seconds later. I believe the cause was extremely dirty power supply fans. I’m glad I took it apart so I could really clean everything, but in a pinch try blowing compressed air into just the bottom, front & back, while using a vacuum at the other end. That would probably have done it.