Power ?
Power-supply is nominally the simplest part of the CPU environment, but also the one that can mess up nearly everything (or anything) when it is having a mood.
Power is suspect for faults at initial startup time because the disk drive(s) tend to be busy a lot. Thermally triggered faults... when room is cold or room is hot, may also indicate a power problem. Some machines will work perfectly at one room temp and fail miserably when a few degrees cooler or warmer.
Power supplies wear out from passing time, from overheating, from surges on the supply source power caused by motors, etc. They can be weirdly intermittent in effect and habits.
For that, substitution of the supply is the simplest course. Can often be done using a loaner or spare supply without removing things that are bolted down - if the replacement is balanced precariously for duration of test and the original left where mounted.
One caution is to be careful to not flex the motherboard too very much when removing the (often stiffly fit) main power connector. Supporting the board some when re-inserting is also wise. Bending the MB a lot causes hairline cracks in circuit layers. Then you pull all your hair out trying to locate them.