Chosen Solution
Hi. I got a second hand iMac. The machine boots OK, i could install systems (tried 10.11 and 10.13). The Mac works correctly (seems), as long as I don’t use a USB scanner or try to access the internet. When I do, I get all sorts of weird things: black screen and reboot (also happens some time when just using software), grey screen (most often), blue screen (once), other screen colors - in any case it freezes and will only boot again after resetting the PRAM (several “sound + reboot” for about 20 seconds). Replacing RAM did not help. I have to decide if I try some repair or break the machine into spare parts. I think it’s a board problem, but which board?
With no error its a bit of a hunt and peck ;-{ I would start off with the power supply and then you’ll need to look deeper into the logic board circuitry.
