Total NOOB question here:

What are the factors that cause frequency to be adjusted down?

Setup: R606 running through Win10, powered by GekkoScience DPS1200 PSU.  My PC is quite powerful with an i7 3.4GHz quad-core CPU and 32 Gigs of RAM.
Settings: Voltage setting 5, and I set --gekko-r606-freq 643
Results: I'm getting about 840-850 Gh/s on average.

If I try to increase frequency to 750 or higher, it seems to auto-adjust down to something below 700 most of the time. I've tried various voltage combinations.

I'm trying to see if I can get it to 1 Th/s average like some others have. Would using a Linux controller make a difference? Is the USB interface on my PC the weak link?

I'm new to mining, and still trying to learn the basics of the electronics.