I installed Linux on a G5 quad. I would expect the iMac to work as well. I think I installed Alpine, but I haven’t started it up in a while. Produces a lot of heat. You’ll need the big endian PPC version btw.
Edit: That screen is not a G5. Linux will run just fine on the Intel macs which that screen is showing.
Do you have any usb devices or other hardware attached? Do you have any background scripts running? Try running without those installed. if possible. If you copied your home directory, try a clean user.
For example, I have an external light meter I made from instructions online and the script for it was accessing the brightness of the monitor with ddcutil and causing a similar spike. I rewrote it to use kscreen-doctor.