Well, I think this is some sort of legendary problem- it's appeared numerous times to numerous people, and even a thorough debugging hasn't helped me fix it. The basics are the usual: selecting any DASH stream gives me a 10 second video initially. After I let MPC-HC buffer a while and restart the video, it gets to some length depending on how much it has buffered. For shorter videos, it gets to the full length by one restart, for longer ones it takes two to three. Here's my current settings:
MPC-HC 1.7.7 5th October, non nightly.
LAV Splitter
LAV Vide and ffdshow Video. I switch depending on whether I want smooth video or not, especially if the Youtube video already is 60fps.
EDIT: After messng about a bit, I ran into another problem: MPC-HC stops playing the video ~60% in, audio continues.