Hmm, I can suggest reseating the card, It will probably not solve your issue but you should try.
Do you have any warranty on the GPU? If so ask them (AMD or the company that made the PCB and cooler) about RMA.
I have an MSI 390x and it does the same thing. I notice it will do it even if there is no screen activity but programs are minimized or running in the background. I looked at the AMD driver side and any time there is the slightest activity to the GPU the mem clocks pop up to 1500. Other times it seems to settle a bit and less of the up and down activity. This is sorta funny as not too long ago the driver would get the card stuck at 150 mem clocks. Im sure AMD will update a driver and perhaps address this. But so far as i can see it is working as intended. Im watching it now and if i type or move the mouse it jumps around but if i do nothing it tends to settle. YMMV.
This also 'seems' to effect it: Turn off 'Surface Format Optimization', set 'Shader Cache' to 'off'. Turn off "Power Efficiency". These setting are located under the AMD Radeon Settings, Gaming then Global Options.
The key thing is when the GPU clock moves above 300mHz for any reason at all(I noticed in the screenie it was 312 at the time) the memory clocks will pop up to 1500. There is no in between. One or the other. Even the slightest movement will make it happen. Unless the card is malfunctioning or overheating I would say its a normal thing. It just the card clocking down to save power and lower heat.
Im not sure what you mean by a clean boot. For some of us it means different things. Do you mean Safe mode? What do you have run at start up ? I'll suggest you may have something in start up that can cause this when 'idle'. Again only some suggestions. I do not think the card is at 'fault'. I think this is just normal activity when the GPU senses any type of work load.
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.