Coppersens,
Exactly the same symptoms on my system. The huge drop, usually but not always followed by higher interim, and finally back to normal - 30-35 drop to 10-15, back to 19-25, then finally to normal. I have 35 fps capped in Nvidia CP.
Running:
6700K; 16GB; GTX1070 to ASUS PG279Q Gsync; 100Mb internet; Samsung 970+ 1TB; Settings mostly high or medium.