May not be the PSU
Could be a number of things
1) Firstly, try resetting the BIOS by either A) Removing the battery from the Mainboard for 1 minute or B) using the Jumper on the motherboard...(your motherboard manual wil show you where it is

)
2) Try removing a stick of RAM and try 1 stick on its own, then try the other stick on its own..
3) You already tried removing a Graphics Card...did u try both on their own?
4) Try reseating the CPU (Make sure you use correct amount of thermal paste between CPU and Heatsink) and check CPU fan is working correctly/clear of dust etc...
Report back after that !!
I dont want to scare you, but I had similiar problems to you (lagging, freezing etc..) with an AMD3200+ CPU system and it turned out to be a damaged CPU
