iMac stops booting shortly after reinstall

2015 iMac. Became extremely slow to boot, suspected failing hard drive, so replaced the drive. Reinstalled the OS from a USB stick, and the machine booted normally a few times before reverting to the old behaviour of hanging at various points during the boot process.

After 90 mins of trying, finally got the machine to boot via USB. At that point I reinstalled the OS once again, and the computer behaved normally for a few reboots before going back to the same behaviour of hanging at various points in the boot process. Doesn't matter if attempting Safe Boot, Recovery Partition, Normal Partition, or USB stick. In all cases it hangs at around 80% of the progress bar. If left for excessive periods of time, it will sometimes go right to the end of the progress bar, but never presents the login screen.

Verbose mode often shows timeouts related to "busy timeout[x], (60s): IGPU"

Any idea why the computer will boot normally initially, and then return to hanging at the 70-80% mark after a few startup/shutdown cycles? It's almost as though some drivers or system files are becoming corrupted after a few cycles? Help!