I have an R9-390. (WHQL 16.9.2 Driver).
No Overclocks here by me. All GPU settings in Crimson are default .
When viewing Global Overdrive in Crimson, I've noticed that every now and then, I will see the Memory dial shoot right up to maximum to 1500MHz (Max default set by Crimson), then instantly sets back to the minimum default setting of 150MHz. This is when PC is at idle. I've noticed this happens on any of the Drivers I've installed over the months. Why does it do that? Is this normal behaviour?
It's normal behaviour. When GPU clocks, usage and fan speed are being monitored in in Radeon Settings/Overdrive, this can cause brief fluctuations in clock speed.