Yess, i have the exact same problem. Fps drops/fluctuations are visible especially in indoor areas. The fluctuations are huge and annoying as hell.
I tried everything from changing all Crimson setting to turning Fallout 4 v-sync off to doing a complete DDU uninstall and reinstall of Crimson drivers but
nothing helped. Turning v-sync off didn't help at all.
I've noticed my GPU isn't running at max clocks when playing Fallout 4. The GPU's core and mem clocks fluctuate wildly.
I reverted back to 15.11.1 and it's working fine now.
I was so happy for Crimson drivers (GTA V runs better than ever with Crimson) but this bug is too much because i play F4 a lot so i had to revert
My system is:
MSI R9 390 8Gb
8Gb DDR3 RAM, 1600Mhz
750W OCZ Fatal1ty PSU
If I could offer any advice it would be to revert back to previous drivers as Crimson isn't too stable right now I'm afraid. I myself had to revert back to 15.7.1 major power consumption issues with the Fury X being capped to 600mhz core. I hope this helps.
Use ClockBlocker. It's a program that some dude made and it fixes these downclocking problems with Crimson until AMD release fixed drivers. Most ppl use it i think because it works great. I use it and have no problems with Crimson.