Assuming the preamp wiring is okay (check the battery connection too), the obvious places I'd check are the guitar's output jack and your cable, all the way through the connection at your mixer or amp. We had a similar problem with a microphone used by the church band; turns out the cable's ground connection had separated so that slight movement could cause it to cut out then go back together. I resoldered it and it's good as new now. Hopefully yours will be an easy fix too.