Yep, mem clock will jump up and down from 150 to your max setting, 1600 in my case.
Doing absolutely nothing. Have this on 2 windows installs and have had this on multiple windows 10 installs, which were needed due to a failing new hard drive.
The 390 will just do this and I suspect it's a driver thing,
Anyway, it's normal.
unless, when it stays at max clocks for a long period. It should be instantaneous jumping up and then down again.