It could be a heat related issue, either with the router, modem, or PC, make sure the router and modem are both vented properly and are not being obstructed by paper or anything.
Run HWMonitor from cpuid.com and record the temps before the game, and while running the game, keep going till it becomes unplayable, then post the temps.
If temps check out, and your hardware is sufficient, the only possible causes are internet, or, something is running on your PC while you are in game.