I think it might be a heat issue. I've had this problem using my AW2518HF monitor with my refresh rate above 60Hz just either eventually starts blinking in game or when stress test. I noticed that when my computer was at a really cold start, blinking would take longer to occur. I decided to undervolt my GPU and increase the fan speed and that helped me out a lot. It hasn't blinked so far but I only just figured this out yesterday so who knows if it is actually a fix but it has definitely helped me.
I have a dual screen setup with one 60hz and another 240hz
GPU: XFX 580 GTS Black Edition
CPU: R5 1600
Memory: G Skill 3200mhz CL14
Mobo: ASUS B350-F Gaming ROG Strix