Well I have a shiny new 600w PSU, I randomly tried out Quake 4 and there it is, stuttering and spluttering along just as before. I can see myself replacing literally EVERYTHING before this gets fixed. I think only my mother board and CPU have stayed the same since I've had these problems. 
I'm sorry but THIS is why people stay with consoles for gaming, and unfortunetly now I totally understand why. :\

I'm sorry but THIS is why people stay with consoles for gaming, and unfortunetly now I totally understand why. :\