Xfinity Stream App on Roku sometimes has picture FF with sound at normal pace, so picture is out of step with sound

I have a Roku Ultra and the Xfinity Stream App connecting to my Xfinity gateway. Several times a week (most recently twice today and once yesterday) while playing a recording the picture will suddenly go fast forward (my feeling is 3 to 4 times normal play speed) while the sound proceeds at normal pace, so the picture and sound are not only out of sync but within a few seconds the scene is totally different from the sound. It happens with different channels, different shows, yet other recordings from those channels are fine.

Sometimes (not always) the two will sync by skipping forward a few seconds and hitting Play, but often that doesn't work, and when it doesn't work I can sometimes find it on Video On Demand. When it isn't available on Video On Demand (none of the diginet channels like MeTV or Antenna TV have VoD copies), I can usually play the recording through the Stream Portal on the Xfinity website and it will play normally, but that means connecting the image to the Roku with the sound coming from my PC, which means I can't use my PC for something else because that would overlay the screen and that gets sent to the TV on top of the show.

The fact that recordings that the Xfinity Stream App that has video issues playing on the Roku plays fine on the PC sounds like the Cloud DVR recording is correct but something in the Xfinity Stream App is triggering a video FF and getting the video way out of sync with the sound.

As you can imagine, this is frustrating for us when it happens.

This problem has never been observed by me for Live TV nor for Video On Demand, nor have I ever seen this on the XG1v4 DVR (not even when it has to fetch the show from the Cloud DVR).