Chosen Solution
iMac works when I press the shift key during start up, which then opens in Safe mode, but during a normal startup, after the apple logo disappears and leads to the login page, instead of the log-in page, a half blank white screen, with rainbow pixels distort half of the blank screen. The iMac works as expected during safe mode as the login page appears, seems normal as well as the desktop after login.
Does the last four characters of the serial number contain one of the following groups (for example, xxxxxxxxDHJQ): DHJQ, DHJW, DL8Q, DNGH, DNJ9, or DMW8 DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, F610 The serial number can be found on the bottom of the stand or under “about this Mac”.
Sadly, your GPU boards VRAM has problems. When you go into Safe Mode you are using the low resolution mode of the GPU which uses shared RAM. When you normally startup the system starts off in low res then switches into normal mode leveraging the GPU’s VRAM.
