When turned on and working, the phone suddenly goes black screen.

I have an iPhone 5C. When i turn it on, it shows apple logo and stays there. Evere 40th time i reboot it, it boots up normally. When i lay it down and not use it for 30 minutes with full battery, it goes black. I have to restart it again, and then it stays on apple logo.. Whats wrong?

If i give it power directly without battery its the same. Ive replaced the front camera cable and battery. Whats left to do? I have a friend who is expert in soldering on motherboards, but we just need to know where to solder.

Any ideas?