I have a 2006 Yukon XL 2500 with the same problem. It originally happened last winter/spring, but went away around the end of April or May, It came back in Mid November. During the summer when it was working, it would lose signal very easily, but it usually regained it right away.
I have a couple of thoughts, but they are speculation...
1, it is connected to daylights savings time....There is some internal clock in the radio that can't figure out what the correct time is and therefore can't find a satalite signal. The signal seemed to come back in the spring around the time the clocks swiched and lost signal again this fall when we changed.
2. There is some kind of internal battery in the xm unit and it went bad. It started dying last winter during the cold and when it warmed up for the summer, it began to hold a signal again. Now that it is cold for winter again, it can't remember anything and therefore can't buffer any data.
Again, these are both guesses and I am on here for the same reason as you, to figure out how to fix this.