After battery replacement, phone wont charge, periodically shuts down
So I replaced my aging iPhone6 battery with an iFixit battery replacement kit. Followed each instruction meticulously. Took me about 90 minutes. With the new battery installed, the phone had approximately 50% battery life. From here I continued to follow instructions to use the phone until it was below 10% before a full charge to calibrate battery - no issues. The next day I thought I would test to see how well this new battery performed. I unplugged my phone in the morning at 100% and used it all day (to my surprise) playing games, surfing the web, watching videos and the battery held up! No issues whatsoever. Later on, when I plugged it into the charger was when I started to see some problems. after about a minute of continuous use, the screen would flash and then the phone would shut down as if the battery just became disconnected. The phone now will only hold steady at 20% charge and is not usable. Did I receive a faulty product from iFixit?
I have tried reopening the phone, inspecting the connections, disconnecting and reconnecting the battery, however, this didn't resolve the issue. Is anyone else having this problem?