I had the same issue. On my old bike I had gen 1 Stages and the Edge 800. Worked without any issues. Bought a new (used) bike with gen 2 Stages and kept getting dropouts etc. Tested with my brother’s Edge 1030 and that solved the issue. So then off course I had to buy the 1030… Worked well since then.
Think I read somewhere that they improved battery life on the newer generations by reducing signal strength. Your 820 is newer than my 800 so one should think the receiver has improved as with the 1030, but perhaps the smaller design has not allowed for this.
Suggestion? Buy a different bike computer 