Asrock 985GM-GS3 fx 785G AM3+ << Bad choice
AMD am3+ fx 6300 six core << Bad choice
8gb ram << OK
coolmaster 400W Power Supply << Bad choice
MSI GTX 1060 6GB << Excellent
corsair watercooling Hydro series h60 << very good
The FX-6300 which needs to be overclocked to get it to lower than average performance from bad performance, is a high power need CPU. The motherboard is in old design with low quality audio, but normally not the issue other than its poor features.
An FX-6300 in gaming, when overclocked, is about as good as a 4th generation core i3 Intel CPU and is priced very cheap. It is much better in other pc functions.
The power supply can be any of many CoolerMaster 400W power supplies and is the very minimum for a GTX 1060 6GB + FX-6300 and is pressed to its maximum when gaming.
B series RS-400-ACAB-B1 (ver. 2) made by CWT, 33A@12V 80 Plus white efficient 230V - Average for 400W
Extreme Power 2 RS-400-ACPB-M3 made by CWT, 18/17split rail. Not efficient. Below Average for 400W
Elite Power RS-400-PSAR-J3/-PSAP-J3 and RS-400-PSAR-I3 /-PSAP-I3 by FSP or Solytech are low quality 350 watt power supplies, labeled as 400W
Extreme Power Plus Tomahawk RS-400-PCAR-A3 /-PCAP-A3 is low rated 400W madeby Acbel
GX II Pro series RS-400-ACAA-B1 made by Enhance Electronics is average 400W
MasterWatt Lite MPX-4001-ACABW is similar as an average 400W
You did not state your country and the maker of the PC. You paid a lot of money for a great graphics card and they filled the rest with below average.
You could have an overheating problem for weak air flow.
Your varying frames per second is probably the underperforming CPU designed in year 2012.
You could have unstable power.
I don't like the motherboard and it is not a good one, but it looks like everything was done to minimize the cost after a great graphics card was chosen.
Although updating drivers could be a fix, this build is in line for trouble its whole working life.
There is too much graphics power for the weak balance of the build.
It could also be caused by software running in the background.
It takes a more expensive set of parts to give proper performance to a GTX 1060 6GB.
A good builder would not sell that PC. I look at the part set and it makes me sad. I have 32 years in Engineering in computer hardware and I don't like when a PC builder takes advantage of a customer.
The motherboard is not good at support of overclock on the CPU.
Gaming optimizing software might help. With the FX-6300, Razer Cortex might stabilize the PC
If you named your country, I could try to suggest a good power supply.
The XFX brand is very good. EVGA supplies made by Super Flower are excellent
Most Corsair are OK or good.
Any made by Seasonic are generally very good.
The 400 Watt Cooler Master ones are either just barely passing or inadequate.
You should have a good 600 watt 80+Bronze or better, but can use a decent 500W as about minimum to be stable.
Yes, a better motherboard would be nice, but with an FX-6300, all of the motherboards are of old designs.
AN FX-6300 is a US$55 CPU.
AN i3-8100 with a new technology motherboard uses DDR4 ram and costs more, but really is about minimum for a GTX 1060.
You cannot fix that PC since it needs a new better CPU, new motherboard, new power supply, possibly a better computer case with good air flow, new ram, and not sure if Windows 10 will transfer license.
Maybe Razer Cortex or other game optimizing, overclock the cpu, check temperatures maybe better system fans, better more stable power supply, and although it will dip in frames per second, it should do a lot better than it is doing.
This is a long answer because the build is so messed up.
Make sure your video cable is connected to the graphics card and not the motherboard.
I'm sorry there is so much wrong, but I tried to think of every fix.