yikes. 350w for a 1060? I personally don't like them numbers but you are running everything at stock settings with no manual overclock on either CPU or GPU? It doesn't look like you are thermal throttling with those temps but maybe you are reaching your power limit not just PSU but power limit settings on the GPU (i.e. how much it is allowed to take in before it has to dial things back)
when you notice your card dropping it's clock down to 1500 do you see a performance drop in your games?
using MSI Afterburner you can increase the power limit and then click the little arrow next to it and prioritize temps over power and see if that helps it to maintain it's boost clock. as it stands though I think it might be that it tries to boost clock up to 1800 but it can't get sufficient power from the PSU and it ends up dialing it back to stock. going up to at least a 450w PSU would benefit you in this matter. But try the afterburner method first just in case it's not the PSU.
If you don't use afterburner or haven't in a long while the new UI for it kinda sucks in my opinion and is a little too fancy and complicated. Switching the skins to the older version in settings would help you to read what you are messing with more effectively.
I have provide a screenshot below to show you what I am talking about with Afterburner.
upload photo link