MBP 16 M5 Pro: QuickTime video stalls on external display

On a brand new MBP 16 M5 Pro, I'm reliably experiencing five second video stalls (audio continues) in Quicktime playback every 90 seconds or so when playing an HD video (generated from FCP) on an external HDMI display. The Quicktime video plays fine on the MBP screen (I played it on the MBP first so it's not a caching issue).

The video file is located on a TB2 SSD. Both SSD and HDMI monitor are connected directly to MBP. This same setup of external drive and display did not do this when I was using a 2018 Intel MBP.

Is there a log somewhere or diagnostic tool to find out what's happening?