I'm new here and I have recently purchased this laptop and been having similar issues. Thanks so much for your work to date - very helpful to me so far.
What I have found to fix the 0.4GHz clock issue is the following:
1) Bios 208
2) clean install (from new boot media, not a reset)
3) windows updates (notably one from ASUSTek labelled "firmware")
4) Chipset drivers Chipset_AMD_W10_64_V2520141501 (run the install.bat as admin - from asus website)
5) Nvidia driver 441.87-notebook-win10-64bit-international-dch-whql (from nvidia website)
6) Armoury crate service ArmouryCrateService_W10_64_V258 (ArmouryCrateSetup.exe - from asus website)
The 0.39GHz problem goes away for good and I've done it twice in pursuit of my real problem which is terrible gaming performance. This laptop should be getting ~11,000 in firestrike at 1080p and I was getting less than 2,000 consistently even with CPU clock speed problem fixed.
Digging a little deeper I find that, it is using the 1660Ti chip BUT the 1660Ti chip is only running at 300MHz according to furmark, afterburner and armoury crate.
Why might that be? According to fur mark, power limit 1 is hit even on an 800x600 run pushing out ~25fps at 100% GPU load. Temps in the 50C range i.e. not hot and fans not even kicking in.
I reckon therefore that there is a firmware power limit bug on 208 (tried 207 and 205 also, same problem).
I'm torn here because the laptop should be pretty kick ass and it's just a firmware update away. Do I hang on? It clearly used to work in the past because of all the good reviews and benchmarks. Any tips for getting this through to ASUS tech support? Can't be bothered with the usual customer service BS.