pc keeps restarting and having display problems

[grass]

so i got a dell optiplex and put it into a new case, then added a new psu and older second hand graphics card, the pc was worken fine before i put the new stuff in even after the psu was in it was working, then i added the graphics card drivers then the pc froze when the driver was installing on 95% then after that the issues came so im guessing its a corrupt display driver, but the pc isnt stable, i cant restore windows to an earlier point of time because it will just crash. what can i do???

specs

i5 2400k

dell optiplex mobo

6gb ddr3 ram 

msi radeon hd 7850

 

oh yeah the cpu wire has an extension i got off amazon on it and the gpu was running off 2 molex to a 8 pin ( ik its a bad idea) and the gpu is a 6pin so i just plugged in 6 of the pins leaving 2 out and i took the gpu out and im having the same problem