Horrible game lag after 20-30 min in game

NVIDIA GeForce GTX 760 graphics 16GB RAM i7

CPU is not the issue..

Ping ~ 90 always has been and never made any issues FPS ~ 160 - 180

issue is that after 20 - 30 min in game lag happens and from that point i'm essentially AFK it's that bad. during lag - Ping and FPS stay consistent... I run the game low quality for higher performance, but obviously that doesn't work.

Any help would be huge.