If a power supply is used that does not provide the required power, the result can lead to an unstable or unbootable system.
I have to say that I raise my hat to you, for being able to get anywhere with only a 300 Watt PSU! Yes, the PSU could really be the cause of all your problems and, if you look in the User's Manual you'll notice that Gigabyte recommend a PSU of at least 500Watts. They state: QuoteIf a power supply is used that does not provide the required power, the result can lead to an unstable or unbootable system.
Ah, the joys of keeping up with technology. Endless research and bottomless pockets are two of the main things you need.