Many thanks for all the input folks. In general when I try to sing and play the guitar, the guitar part is fine. It seems, however, that my ability to hold a tune vocally just vanishes as soon as I pick up the guitar. Even songs that I've sung since I was a child and have sung well unaccompanied for my own children- put a guitar in there and the wheels just come off. I understand the importance of practice but there seems to be a problem with the way my brain is wired. No guitar- I can sing. With a guitar- I cannot. Why?