I really think its the motherboard.
If you turn off the power and back on and it works, it seems that the motherboard has an issue with the power delivery of instant on/off without a drain.
It shouldn't be the memory or the CPU, as the only thing that takes power when you shut down and back up without fliping the switch, is the motherboard.
To make sure of that, just start the computer with one ram stick at a time and see if you still get that issue, or just run linx for an hour to make sure the CPU is stable.
This could be that only now the problem has appeared, and for a long time up until now, you just hadn't done any "hot restart".
If you can live with this for awhile until you upgrade next, its fine.
But mind you that this might degrade after awhile.
I used to have a faulty motherboard, which at some point just would go on and off a few times until it was able to actually start. Its not really fun.
Anyway I hope you will be able to pin-point exactly where the issue is.
At least you are learning as you go