This sounds like something started running in the background on the computer.
Whether or not this is related, everyone needs to be reminded, when running Windows 10 and an update comes in, it WILL install during a game and can cause everything to come to a grinding halt until the install is done.
In all other versions of Windows, you should set it so you can run it manually. Windows 7/8 systems got some updates which mimic a lot of Windows 10 data gathering abilities which also cause uneven system performance. That particular service can be removed from the computer with nice side effects.
Alt-tab, if you can, from the game, then check to see what is running in the background.
Stutters are different from an extended drop in frame rate and we still have not been able to duplicate the stutters some (I say "some" as the reports are very sporadic) people are having.