Hi, it's a know bug, memory leak. I have:
*i5 4670K (OC to 4.4 ghz)
*Asus GTX 970 4Gb
*8Gb RAM 2133 Mhz
*Windows 10
I have the same issue that you describe, Vram will slowly but certainly increasing, making the game stuttering like crazy and finally crash. I temporary fix this by increasing the Windows Page File, original was about 8Gb and i set it to 20Gb, as recommended by others users. With this, i'm able to play multiplayer for 2 hours without crashing. In addition, enable both flat shadows options, and decrease the preload radius (i set mine to 80 000), hope it help you.