Your computer hangs after few minutes, because it can't properly enter sleep mode. Usually it is kernels fault.

Temporal sollution: Go to System Preferences, then Energy Saver. Move the slider
with a description 'Put your computer to sleep when it is inactive for:' all the way right to the position 'Never'. That way your computer will never try to enter sleep mode. Don't touch the other slider, 'cause its better if your display turns off when you are away - your monitor will 'live' longer.

Long Term Solution: Search for a kernel more proper for your hardware. Give a spin to a StageXNU kernel aka Voodoo kernel. It's still in works, but has a lot of potential.