i just recently installed the RX 480 and got major lag spikes in various games like Fallout 4, Overwatch, DayZ and War Thunder.
The games runs on 60 frames the first minute at the beginning but then immidiately drop down to 10-3 frames every 5 seconds.
BenQ RL2755HM 1920x1080 using HDMI [60Hz]
Do you have the Power Limit in Overdrive set to +50?
Do you have your computer power plan set to "Performance?