Good afternoon Mark!
I have had this exact same thing happen to me on an iPad 6. I had isolated the battery through the repair and when reassembled it would not pull amps with the screen off and would restart every 2-3 minutes. I took it back apart and removed the motherboard to inspect the battery connections. To my surprise, there was a small spec of dust preventing one of the contacts on the board from getting good connection. Removed said dust, reassembled, and the device has been working ever since. I cannot guarantee this is your issue, but it fixed the one that I had!