Looks like that system uses NVidia Optimus tech. It's a newer attempt at gluing an integrated graphics solution with a more powerful discreet GPU, without the hardware multiplexor to redirect video output.

Can you run GPU-Z against the integrated graphics at the same time, and see if those stalls are from the NVidia GPU trying to hand over processing to the IGP?

Or, you can also look to disable the Optimus feature, and try and force everything to the NVidia GPU at all times and see if the problem persists.