What could be causing this rendering error?

Hi all. I’m experiencing a strange error when rendering. I’m editing an MP4 and rendering using the YouTube preset. The first render appears to complete correctly, however when I play the resulting file, it plays correctly for the first 5-10 secs, but then crashes. I re-render…and the same thing happens. Same thing two, three or four times. And then the render is successful. Could anyone please suggest while this behaviour might be happening (and what I could do to improve the render reliability)?

Many thanks and warm regards, Bill Hester

DR 18.6 (free version). Windows i9 12th gen, 64GB RAM, Nvidia RTX3070Ti graphics card