Keeps restarting and hit issue!

[boombd1]

i bought this phone second hand.. Its a sprint unlocled
Phone looks brand new.
after using 30 minutes i find heat problem
heating issue is too much i mean it feels like sometime its gonna explode after using yotube and multitasking all together.

so i tried to root and install unncesary apps that casue this extra drainage from sprint apps.
When i flashed it syas passed.. so the flash was Ok.... but now i see it keeps restarting .. restart every 5 minutes now...

So whats the problem here??
i factory rested...

any solution?