I have the problem sometimes of my nexus 7 losing battery while charging sometimes as well, but I never use my stock charger and don't know where my original micro usb cable is. I have found that different Micro USB cables can have a major impact on charge rate. When the Galaxy S3 came out I noticed that with the stock charger and stock cable, I lost battery anytime I used my phone even while charging. My suggestion to you is to install the app current widget and put a widget on your home screen to see the amount of charge your device is receiving while charging. Yes you cannot do this while in a game, but you can at least test out different cables and chargers you may have to see if maybe one of those hardware pieces is the culprit.
Slappin my rooted Moto X
Rockin 2013 Nexus 7