I have a 1070 in a PCIE 2.0 x 16 slot on my machine I use daily. The clock typically boosts to around 1962. However, there are times it drops to 1600. This has happened with some web pages open (I think when certain content includes video) and sometimes Visual Studio. If I close the offending applications, the clock goes back up to 1962'ish. However, the entire the GPU usage stays around 96, but the power % drops proportionally to the clock. Not sure if this is your issue, but it's what happens to me.