am getting a "boot freeze" from time to time, I noticed this when doing some power tests so that I can know if the system would be able to boot after a power disruption (yes we have alot of those here & UPS backups helps but not much, sometimes blackouts stretch for hrs). Its unpredictable problem, sometimes it happens sometimes it doesnt.
The most common reason for this I have found is bad ram, with a bad power supply a close second. I would start by running memtest86+ overnight. This makes sure you have a stable platform first. If that finds noting, try swapping out the power supply. If it is 12v, try a car battery and see if you can replicate it. It could be the PS or the poor power you know you have. I know a guy that built a continuous power supply with a car battery and trickle charger.