Wants to turn off by itself?

Hello all,

I Bought a cracked iPhone 6 plus for repairing. It acts strange, because sometimes it wants to turn off. The screen where you "slide to turn off" comes on random sometimes. It acts like the powerbutton is being held in, but this is not the case as i have disconnected the power button cable.

When its off it can sometimes turn on as well. There is no waterdamage, its just dusty inside, and the battery looks ruff. But i dont know if the battery is the problem?

Thanks!