ASUS X552LD Lags when Plugged In

Hi , I have recently purchase a new ASUS X552LD on 7/6/2014 . It's just out of the box . Okay , First of all , When I play games like Assassin's Creed 4 on battery power , It runs prefectly fine , However , When I try to charge my battery and continue playing , I get a noticeable framerate drop . The framerates jumped up and down , When I remove my A/C supply , and run on battery power , It runs prefectly fine . I have updated my NVIDIA Geforce GT 820M . Set my default GPU as 820M instead of Intel HD graphics . I have changed my power plan settings , But none can fix this problem , I wonder if you guys have a permanent solution for this . Because I have googled about this problem and tried every single solution , But none can solve my problem . It is getting really irritating for me to charge my laptop every single time I try to play games . I am afraid that It'll reduce my battery life . I tried removing the battery , and use the A/C as a power supply , It is still the same . No difference if I plug in the battery and charge . I don't know if the problem is my Power supply adapter or anything else inside my motherboard .
My Laptop specs are:
Intel i5 - 4200U @ 1.6Ghz Turbo boost to 2.6 Ghz
4 GB DDR3 RAM
500 GB Toshiba
NVIDIA Geforce GT 820M 2 GB@720Mhz / Intel HD 4000 series