Hi,
1- monitor your cpu and gpu temps (hwmonitor and core temp) to rule out overheating.
2- Download ans install msi afterburner and activate in game cpu / gpu monitor to see what % the utilization of each.
if CPU is at 100 % and gpu low usage, most likely means the gpu does not have enough power / defective or bottle neck.
while GPU 100% and low gpu
Also, pubg might of have a update that impact resource utilization. They mighty fix that later on the way since it just came out of beta.