DVR playback on TV corrupted, but fine when streaming

There have been many posts about flaky DVR playback, but I haven't seen a post addressing my specific version. The issue below has been happening the last few months and occurs with 90% of the programs we record. I would call support, but it's really difficult to get a human being, much less someone who is technical enough to figure this out. So, I'm trying here.

Here's the problem - I record a program on the DVR:

1. TV playback is corrupt: When I play it back on a TV, the recording will have moments when it's pixelated, scrambled, jerky, skips, rewinds a bit, and sometimes it just stops. Every time I play the recording on a TV the exact same errors occur in the exact same locations in the recording. 
2. Stream playback is fine: When I play the same recording through a streaming app (Roku, mobile app, smart TV app, etc.), the playback is flawless.

I've been told that the X1 DVR box has no internal storage, but the only explanation I can imagine is this:

1. Maybe: The box pulls the recording from the cloud and stores it locally, corrupting the local copy in the process.
2. Maybe: When I play the recording on a TV, the local corrupt copy is played.
3. Maybe: When I play the recording via streaming, the clean original version is played.

Here's what I've tried, none of which has helped:

• I have replaced the main DVR box 3 times and our one satellite box once.
• An Xfinity technician came out and checked all cables and connections. He said all the signals were strong.
• I've rebooted the DVR box multiple times.
• I performed a Sync DVR.
• I performed a System Reset.

I wonder if the next steps are:

• Borrow an Xfinity cable modem for a week or so and see if that makes any difference (we use our own cable modem and wireless router).
• Have another tech come out and recheck the cables and connections, but maybe more thoroughly this time.

One other note: The errors seem to happen most often if the program we record is in primetime. (I haven't tested to see if there's any correlation with whether the program has Smart Resume)