Make sure your PC is un-plugged from mains power before trying this.
Have you tried removing the GPU from the PC resetting the CMOS just take battery leave for a min before replacing.
And then booting the PC with just the IGP on the CPU?
I had an old PC that done something similar. it would have blue and white lines going across the screen while being in the BIOS of the motherboard sometime the picture while being in the BIOS was fuzzy with kind of a red hue you could still read everything in the BIOS and it still work ok.
It didn't always do this just sometimes, I had tried changing the graphics with 3 different cards I have spare, but they didn't make any difference, I tried changing the ram with different DDR2 Kits I have, still no difference, re-flashing the BIOS, changing Cmos battery, bare board tested it with just RAM,CPU and GPU, different monitor even a flat screen TV nothing I tried seem to make any difference
Could it be the PSU causing an issue like that?
The PC still works today and still sometimes gets the lines or lines with fuzzy picture while in the BIOS, the lines have only appeared when the PC was sitting at desktop a couple of times.
I have more or less concluded it's most likely the motherboard or CPU failing but I don't know for sure because it still performs as it should.
The PC has an old intel Core2 Duo Processor E8400 and it's on an old Asus motherboard can't remember the motherboard model
It would be interesting to find out what your Issue is because it might help me identify what might be going on with that system (it's been doing it for years now)