GPU Usage drops in open areas with intensive elements causing low fps. (Please Help)

As the Title says, i'm runnng a gtx 1080 Ti, i7 5820k@4.0ghz, 32gb 2133-2400mhz Ram and a 1300watt psu. I'm gaming on a 2560x1080 Monitor. I've narrowed down the issue that i've been having of gpu usage drops. it seems that i'm getting massive drops when processing intensive elements ''only'' in open areas. for example in GTA V and Skyrim. when in enclosed areas i get 99% gpu usage with buttery smooth fps, for skyrim when i enter caves and mines my usage is also 99% but when i go outside my usage drops to 30s-60s. causng my fps to go below 40. my gpu usage seems to reach 99% when it doesn't have to do a lot of work but when it faces elements in open areas it goes down to the 30s giving me really shitty fps. pls help as i had this issue with my previous GPU a 980Ti. it's not a gpu hardware issue.