My computer crashes in different ways

I have an Emachines S2482 with all of the original parts from a few years ago except one upgraded memory stick, a Radeon 9550 video card, and an ethernet card. Every time I boot up the computer it will either: freeze up anytime between the bios startup and when I’m actually logged in, reboot instantly between those same times, or it will boot up but not dispay anything on the screen. If the screen is blank sometimes my green ring of light on the power button is not on either. I’m not sure what is causing the problem or if there is more than one problem. Any ideas?

Edit: forgot to mention that the computer will work fine after one or two forced reboots (holding the power button five seconds).