Wont power on. iPhone 5.

So I had and iphone 5 that i bought with insurance, the original broke and I received a replacement phone. the replacement wouldnt charge so they sent me a second replacement. Well I tried to fix the one that wont charge, I reaplced the screen and home button(I cracked the screen when i first got it)I opened the unit and replaced the chargeport everything turned on and worked great i left it for a couple days came back and it wouldnt turn on. So I swappped the batteries from the current iphone5 im using and everything turned back on. So i ordered a new battery and replaced that as well. the phone again turned on right away and worked. So i left it for a couple days again and now its off again and wont turn back on. I have replaced the screen, battery, and chargeport. Whats my next step?