Question I have something around 100 fps and in full screen lags

[eshherr]

Hi i just got today a new monitor. I plugged it to my graphic card as second monitor, then i unplugged the old one. When i turned on Witcher 3 i noticed that on full screen there are some unplayable lags (i had 70 fps but its felt that i had 20 max). On old monitor everything was ok. Do you guys might know what is the problem here?
Specs:
CPU ryzen 5 1600
GPU Radeon RX 580 4gb
RAM Patriot 8GB 3000MHz Viper 4 CL16
Motherboard MSI B450M PRO-M2
PSU SilentiumPC Vero L2 500W 80 Plus Bronze

 

[rgd1101]

same resolution?
what are the exact model?

 

[eshherr]

Acer Nitro VG220QBMIIX ye same resolution 1920x1080

 

[rgd1101]

turn on freesync?

 

[eshherr]

It worked for 2 hours then IT started to lag again :/