HD Content problems

I wonder if anyone can help me.
I’ve installed Kodi 16.1 on a new firestick but I’m having problems streaming HD content (720p / 1080p).
With both acceleration options ticked under Video Settings, the picture is very juddery and unwatchable.
If I untick the acceleration options, it is as though the video is playing in slow motion with the audio way out of sync.

Does anyone have any ideas on what I can do to fix this?

Streaming SD sources, there is no problem.