I have had a similar intermittent lost LF channel issue after the receiver has been operating for an hour or so. The relay click occurs, then the LF channel cuts out. I have discovered that when that occurs, I can get the channel to return, every time, by mechanically pushing down (with the cover off)on one of the 10000uf electrolytic caps on the mother board. From this symptom I have deduced that the problem is a cold solder joint on the area of the capacitor. I have not figured out how to get to the bottom of the mother board so I can scope it out and fix it.

My amp sits at ear level to my immediate left so the temp-fix is easier than finding the bad connection.