Unstable FPS in LoL

[ithemechanic]

Hello. I recently bought a pc which has :
Processor:AMD FX8350
Motherboard:Sabertooth 990FX R2.0
Graphics Card:ASUS R9 270X 2gb
RAM:Kingston Hyperx Beast 8gb @1866 MHz
When the game starts i have 250 FPS.After 1-1:30 min i have ~130 FPS.All very high settings.All last drivers are installed.Why is this happening?

 

[IRONBATMAN]

It could be be because of the server that's causing you to lag