The bootloader is not the problem I think. It works or works not. Same with software. If it is corrupt it always fails at same point in the code.
If it sometimes works and sometimes not points to a hardware issue. It could be that your power supply is not stable or lacking sufficient power preventing the disk(s) from spinning up. In that case the bootlaoder gives up after some time because it doesn't find a working disk. On nwsp2 you get in that case the red flashing led.
It can also be a capacitors on the mainboard going bad preventing that sata inerface comes up.
Because of the red led behavior I would first try in your case if another power supply solves the problem. Changing capacitors on the mainboard is not so easy..
But please note that I am guessing here on what could be the problem, I can be completely wrong