Mine does that, no matter what cable or plug you use, when you charge it via the the wall plug, it drops power, or at best maintains it. Funny thing is, when you charge it via Car, or through the USB socket on my iMac, it charges no problem.
The problem originally started this week, when I was away in a hotel and woke up to check my phone in the night, only to find it dead and unable to charge.
Luckily I'd bought a spare charger, which worked, but now even that one is not doing the job, nor is my one at home that previously worked before I went away. I have used a variety of wall plus/cables with no joy.
I have downloaded an app from the play store which shows 1200 going in via the wall plug and only 450 via a USB port, so this seems very strange