Are you plugged in while playing or on battery power? Apparently (according to a post in a different forum, which I forget... might be the MSI forums, might not be), if the battery drops below 80% the laptop automatically limits performance. I'd try removing the battery and playing plugged in to see if that fixes it... older and wearing-out batteries can do all sorts of funny [expletive].
If that doesn't do it, the next step has to do with Windows maybe switching to the integrated Intel graphics for no reason. To quote someone: "Try opening the nvidia control panel and under manage 3d settings, setting the preferred graphics processor to "high-performance NVIDIA processor."
I got that from this forum: https://www.reddit.com/r/MSILaptops/comments/64gezf/msi_laptop_fps_drop_in_games/
I know it's a different laptop, but software automatically selecting the integrated graphics does happen from time to time. You also may have to explore your Nvidia options a bit, since they do change the layout of the Nvidia control pannel from time to time.
And, if you're still having trouble, look for the latest stable drivers from the MSI website, as sometimes new drivers don't work out too well. You'll want to do a clean install of it.
If all that doesn't help... all I can think of is taking your laptop apart and replacing the thermal paste; you may have cooked it by accident, and it does become less useful with time regardless. (look for a guide, as the manufacturers can be sneaky about how they assemble it - take pictures at every step and keep screws either in their holes or separated in groups; make notes on special screws or steps if you like). Oh, and while you're in there, look for any obviously bad capacitors or burnt/corroded areas.
After putting it back together, making sure to connect all the connectors again; if you're still having trouble... it may be time to accept that either the motherboard, processor, or GPU is damaged. And while some laptop motherboards allow you to swap processors, the GPU is most certainly soldered to your laptop motherboard... and that is a job for either the bold or the professional, as most people can't afford the proper equipment to do so (the oven "reflow" trick is NOT a good solution unless you plan to scrap the PC in a few weeks/months).
Hopefully you find some of that helpful. I do not envy you; diagnosing electronics issues is a real time sink sometimes.