After this thread, I investigated and discovered my tv has a way of displaying signal strength. I found that the two channels that sometimes fail to record were listed at signal strength of 34-40 (out of 100) by the tv, all my other channels were 100/100. The Tablo gave me 3 orange dots for these two channels, btw. As I went through the channel list, the tv gave me a “could not decode signal” on one of the channels. I guess I need a better antenna.
So… long winded way to say check if your tv can display the signal strength.