For the first one, I assume you changed the power supply unit, right? The twxo things I can see is the video card getting loose somehow, so you just need to take it out and put it back in to check, and the second is to check the video card fan (if it has one), if it broke and you start asking a lot from the card's gpu, then it just poops itself when it overheats.

For the second issue, I have no idea, I never use stand by on this computer, because I can't get out of it.