I had a similar problem with my HD621 and a Pioneer LCD TV. It may have been a combination of the cable (10m) and TV spec. I solved it by putting a HDMI booster between the HD621 and the TV. ( This one in my case) ). Strangely it worked better at the HD621 end of the chain.

Main system - Sources (Kodi-HTPC, Vero 4k, FireTV, Sat) into Trinnov Altitude 32 (8-16) (+Roon), Trinnov Amplitude 8 amp & Vivid Audio (Giya G3, V1.5) and REL Stentor III speakers. Others: F80 + Raspberry Pi and other Pi systems.