I'd reccomend the logitech g203. A little heavier than what you said at 85grams but ive used it for fps games and it feels just fine. I recently upgraded to a g502 which is even heftier at 12 grams and I find even that suffices quite well
You might be able to overclock the monitor.. In the radeon panel, you can make custom resolutions and specify the refresh rate. Most 1080 60hz monitors can clock to about 72hz.
Sounds like your cpu is thermal throttling. When your cpu gets too hot, it cranks its clock speed down to generate less heat so that it doesn't damage itself. I would monitor temps when this happens.