Whow! Same problem here, except tuner #2 was the problem with my HR21-700. You describe almost 100% of what was happening to me. Here is what I found and have yet to see if it will help or not: I noticed one of my B-Band Converters was hot and the other cold. But no matter what tuner I swapped with a converter, tuner #2 was always screwed up. Then, I would actually squeeze the converter or place it in a funny position and things would work on tuner #2. I just installed two converters that Directv sent me today, free of charge and so far it's working like it should. I really don't know if that's what the problem was, but Directv said to try this first and if it continues, they will replace the receiver for me. It's worth a shot to see if you have the same results. Fiddle with those converters and see what happens. Be careful though, I ended up breaking the black plastic cover on one and to make matters worse, the metal covers then came off, exposing the guts of the converter. LOL, weird thing though, is that tuner #2 started to work after I basically destroyed that converter!! 
Also, my cables seemed as though they were welded on the dang receiver. I did get them free and noticed a large amount of some white substance inside the male part of the cables. After I did get my new b-band converters, I tried to get one of the cables off the old converter and being that it was so tight and pasted with that white stuff, the inside of the converter broke off, connector and all still in tact. I never did get that piece of cable off that comes out of the converter and connects to the receiver. I am still trying to figure out what that white stuff is and why it caused the connections to become so tight. It almost seems like some type of electrolysis.
Also, my cables seemed as though they were welded on the dang receiver. I did get them free and noticed a large amount of some white substance inside the male part of the cables. After I did get my new b-band converters, I tried to get one of the cables off the old converter and being that it was so tight and pasted with that white stuff, the inside of the converter broke off, connector and all still in tact. I never did get that piece of cable off that comes out of the converter and connects to the receiver. I am still trying to figure out what that white stuff is and why it caused the connections to become so tight. It almost seems like some type of electrolysis.
