[Q] s3 boots to home screen but flashes continually then eventually restarts topic

Ok so prior to getting the AT&T s3 i747 I had a s2 i727 and on two occasions I cracked the screen and/or LCD and just purchased a broken s2 (water damage..etc) off ebay and then put my motherboard in a new shell basically. I’m attempting to do that now with the s3. The seller on ebay said he was told it was an issue with the motherboard but the screen and lcd were good. I tested it when it first got here and it would not turn on at all. Now that I have switched the motherboards around it will turn on. It goes through the logo screens and starts up fine EXCEPT the screen continually flashes and after a few minutes will restart itself. I messed with the power button but I don’t think that is the problem. When I turn it off and charge it with my battery it does exactly what it should but when I do the same with the battery that was sent with it the battery display flashes on off on off and it pretty much won’t keep the charger connection going. Every time I google it or read a forum its ppl who can’t get past the Samsung screen and I’m having no luck getting answers. So if anyone knows whether this is a screen/lcd/digitizer issue or something internal please help me so I can return the phone before it’s too late! Thanks
Oh and I can boot into download mode but not into recovery mode and I have already reflashed using odin which seemed to be successful..it said pass