Last time I had a G5 intermittently freeze or fail to detect memory, the culprit ended up being an intermittent BGA joint on a large IC (presumably Northbridge/Chipset) on the reverse side of the board. It's below the orange/copper portion of the heatsink in this image:
However, my machine was a Single 1.6GHz Air Cooled G5. I had heard recently of later G5s, including the Dual 2.5GHz machines exhibiting the same problem with the solution being a solder reflow, as I found when I came across this video. One critical piece of information I have to mention is that due to the size and construction of these boards, it is very easy to warp them if too much heat is applied in a centralised location. The creator of the linked video appears to have some success with using an oven to reflow the board.
Of course, your mileage may vary and it's difficult to be certain if this is actually the issue with your particular machine, but the symptoms sounded all too familiar so I figured it was worth chipping in on it, even if you attempt this only as a last ditch attempt.
However, my machine was a Single 1.6GHz Air Cooled G5. I had heard recently of later G5s, including the Dual 2.5GHz machines exhibiting the same problem with the solution being a solder reflow, as I found when I came across this video. One critical piece of information I have to mention is that due to the size and construction of these boards, it is very easy to warp them if too much heat is applied in a centralised location. The creator of the linked video appears to have some success with using an oven to reflow the board.
Of course, your mileage may vary and it's difficult to be certain if this is actually the issue with your particular machine, but the symptoms sounded all too familiar so I figured it was worth chipping in on it, even if you attempt this only as a last ditch attempt.