Maybe you have madvr enabled and it asks very much performance from GPU.
Try to select EVR Custom with D3D Fullscreen. MPC-HC menu View - Options... - Output tabIs it help you?
Unfortunately no, I tried this now and it remains unusable the "High frame rate index" goes crazy 0.3 ~ 2.5, and with many stutterings in the middle of it. One thing I noticed is that when I open a video the processor usage rate is 80 ~ 90%, this with the gpu acceleration on, while the gpu does not reach 30%, after the stutterings begin to use cpu drops to less than 10% but the GPU also almost 0%.