This looks like a ram bottleneck rather than gpu. Gpu usage 99% is optimal which means your graphics card is running at full potential. If your drop quality settings and you face the same issue then you should look at cpu or ram. For gaming you should consider a 2x 8gb dual kit.
Also which i5 exactly and which game? It could also be a cpu bottleneck (depending on the game) but from my experience your ram is not able to handle the traffic or is simply maxing out it's usage and the pagefile is hindering your performance