That makes sense. I may have to look at that. (Covers already been off to put a two TB drive inside)The HR20 physically has two tuners inside, with a cable hooked to each running off the one input. It is a soldered cable that looks similiar to an RCA jack. Anyhow, only reason I can think of for that to happed is the cable has come loose or the second tuner is failing.
I think you are ok to take the cover off and check the cable. The HR20 is no longer being taken back by Directv when disconnected.
Because of the way the two tuners are connected, the performance will not be identical. If the signals are already low, the second tuner can be lower. Also the performance when there is multipath can be different. What signal strength are you seeing on a typical station?