Try the easier first step of setting power management for the game to "prefer maximum performance" before you underclock. Also what temperatures are you getting on your GPU?
Do you use the integrated graphics driver on your cpu at all? If so it might try to switch between them or something. You should in nvidias control panel be able to set the dedicated one to always be the primary one.
I've just had a quick play, I already had the power management setting set to prefer maximum performance, and I've got the Nvidia control panel to always use the Dedicated GPU for everything when I'm not on battery.
It certainly is a temperature issue I think, I underclocked the core clock by 135Mhz (As much as MSI Afterburner would let me) and turned on the temps in the OSD. Usually this card runs in the 88-90 mark, which seems a little high but is pretty much standard for this laptop.
This time however I noticed that once they started to creep to 93 was when the throttle seems to kick in. I've never had this problem with any game I've ever had, although admittedly the last game I played that properly pushed it was MGS5:GZ and that was a good few months ago.
I'm guessing that my next step to try this time would be to take it apart, give it a good clean and repaste the heatsink on the GPU and see how I get on.