My first thought was that it was a bad guitar, but strumming worked fine to move menu selections, and the fret buttons worked fine to select menu options. I thought it might be the Wii controller, but the same thing happened with both controllers. My son wanted a 2nd guitar anyway to go co-op career, so we got a brand new guitar, and the same problem happened with the new guitar as well.
The solution happened to be that we needed to delete our save data.
Hope this helps somebody.