After one month and one day in repair, my g5 is finally back in action. The problem was that it would spontaneously reboot when under moderate to heavy CPU load. I deduced that it was a cooling problem by manually cranking all the internal fans up way high, which made the problem to go away. Observing temperature sensors, I noticed that one CPU seemed to run quite a bit hotter than the other.
They replaced the power supply, both CPUs, and the logic board. Problem didn’t go away, so they replaced the logic board again. Now it’s cool, and the CPU die temps are a lot closer to each other. Those parts would have cost me $2663.12 (were it not under warranty), meanwhile I paid $2070 for the machine in the first place… woof.