Recovery mode Loop/Won't go thru iOS8 update via itunes

Hi Ive recently bought a phone with an "easy" fix, at least i though it will be easy. Phone was working on iOS 7 but guy told me that it keeps restarting all the time. You can use phone for like 5-10 mins and then it just dies. I thought maybe new update will solve the problem but unfortunetly what happened was that only itunes see this iphone but after downloading iOS8 via itunes and trying to update but it just freeze. Also in the middle of software download phone resets like someone would take out USB cable from pc and put it back in. Anyway it stuck in recovery mode(i can put it on DFU but it won't change anything) and keep doing same, even special rebooting programs dont help. I know that volume up/down wasnt working, maybe this cause that issue? Also im pretty sure that guy been messing with the screen as it has flickering effect and sometimes losing colors or weird lines appears on the screen. Touchpad and rest are fine. Ive tried nealy all common solutions for it.

Any ideas my friends?