Question Sony X940E with Popcorn Hour A500 - having a lot of trouble syncing them!

Hello everyone. I've just bought a Sony X940E TV and am trying to set it up correctly with my Popcorn Hour A500 box. I'm experiencing a lot of judder to the point where many things just look unwatchably bad. I tested the same content from the same player on my LG TV (LM7200) and it looks fine. I've tested a variety of things from my lossless Blu-Ray rips to Amazon Video downloads and it's all the same. The A500 is connected to the Sony TV through my Denon 3400H.

The A500 has a "framerate sync" setting as well as a resolution setting (1080p24hz, 1080p30hz, 1080p60hz, 4k24hz, etc). I only use it to watch 1080p content, but it also has excellent upscaling so I assumed 4k24hz would be the best option but it looks pretty bad.

The Sony TV has a Motionflow setting where you can choose "Off" or "True Cinema" - I'm not exactly sure what it does, rtings just says you should choose it if you want to remove judder from cable boxes (they also say the TV should be able to play judderless 24fps content in general). It also has a "Clearness" setting that mostly fixes it but also makes the picture too dark to be comfortable.

I've tested a lot of different combinations of settings on the A500 and the TV and can't figure out what works best. Does anyone have a recommendation? Thanks.