Performance problems topic

Hi guys. I have noticed that my Xperia Z Ultra has some performance problems. This problem is when gaming. It is something strange because sometimes games run very smooth but sometimes it is very laggy (more of the time). I dont know why it is happening. For me its like phone is not pushing feadback to cpu or gpu when its need it. My phone is c6833 it is stock 4.4.4 stock kernel stock rom. Can someone help me please??