If you've tried swapping over HDMI cables, then it could be the internal PSU in the HD621...

Do the lights flicker on the front when the signal drops?

They use the same PSU as the G-Series - XP Power ECM40UT31

Try turning the HD621 off for 1/2 hour and let it cool down - see if that changes anything.

Good luck,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200.2HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5200HC, DSP33
Spare: MCD, 100 Series, 200 Series, 500 Series, F80s, C15s.