I've had the issue (at least) since late VIX 4.x and as well 5.x

The problem description:

-Then viewing of HD channel recording, the viewing starts to jam for second or two every few seconds , when there is some other (timer) recording is running on the back.
-After FW upgrades unit has worked normally some weeks.
-Next assumption was bad PSU. I replaced some suspicion capacitors
-After PSU 'fix' unit has worked normally some weeks.

Now with VIX 5.4.009 (updated 2021-03-31) the symptom hit again, I started to wonder if the problem would be memory leak.
-I restarted the Enigma2 via menu
->The behavior remained, HD recorded viewing was jamming.
- I restarted the unit via menu, keeping mains power on.
->Unit works normally, HD viewing doesn't drop the frames with parallel recordings.