Lumia 925 sudden battery drops.

[Narada93]

I bought a lumia 925 brand new few days back and I'm having a very short battery life compared to other lumia devices. I noticed sudden drops of two or three percentages at once sometimes(like 35% right after 37%). Specially when playing games or running an app. And also i noticed skipping some percentages when charging(like 94% to 100% at once) But this doesn't happen always or at exact percentages so I can't even prove the error to shop people.Sometimes stuck at some percentages for 5-10 minutes even in heavy usages and after stopping the work and keeping it in standby it loses few extra percentages in standby. Tried flashing and reinstalling software but no luck. Is it a serious problem? Or is it normal? How can I prove it to shop people? Thank you