If you recently (forced to) upgrade windows 10 to the Fall Creator’s Update 1709 some time late last year or early this year
And you’re using an Nvidia GPU
You’re likely going to experience fps drops from 60 to 30fps and it stays that way until you trigger a graphical spike, only to be triggered again by another graphical spike inducing scenario (multi hitting, high monster density, ded parts exploding, chat, inventory etc)
A recent february windows update will fix that problem
Though if the update is still not available for you, turning off full screen optimization for the .exe file (right click the .exe and look for the compatibility tab helps reduce the occurrence [you can also turn off some new features in windows accounts setting that you dont really need])