Sometimes it works smoothly, sometimes it's lag is unbearable

[WenDMegs]

To get it started, my spec is:

Intel i5-4460 3.20GHz

Windows 10 64-bit

16GB RAM

NVidia GTX1060 6GB

3840x2160 60Hz

Sometimes it plays smoothly on high settings, sometimes it goes bananas and even lagging heavily inside the Orbiter and/or solo plays, practically makes the game unplayable during those times. Is there anything wrong with my hardware or software?