ATT Uverse -- On Demand Skips, Freezes, Unusable

We have only one television set; for 10+ years, we've had U-Verse TV. A couple of years ago, we were given a new Arris VIP 2250 DVR. The DVR is wired to our AT&T U-verse Pace 5268AC Gateway Router. We also have AT&T Fiber Internet, which is super fast. It’s our understanding On-Demand programming is accessed through the Internet. Problem: We can’t see On-Demand programs because the audio/video freezes and skips. DVR is connected via a new HDMI cable to TV. Any thoughts? (All other U-Verse TV services work just fine.) Thank you.

This problem has existed for years which forced us to move away from On Demand programming. We've purchased movies that we couldn't see. We've called customer service on various occasions and have gotten no helpful assistance to fix this bothersome problem. 

Thank you.