On Thursday my HTPC started misbehaving badly when playing TV3. It started stuttering terribly and became completely unwatchable. I was pulling my hair out trying to sort out the problem. My CPU and GPU usage was minimal. I replaced my system HD as I was a bit suspicious and also tested my Timeshift drive but no problems there. No change to playback. So I checked the MP forums and it was mentioned that setting Prefer AC3 in the TV settings fixes the problem. Set that and what do you know, TV3 is playing back properly again. So before you spend several hours trying to fix your hardware try the above fix.
It would appear that TV3 have changed something with their AAC encoding that can break playback in MP. Anyone else had this problem? I hope TVNZ don't make the same change as they don't offer an AC3 audio channel.