Speedo problems

I have a 2004 VTX 1300C, it just turned 16,000 miles and have never had any issues. Yesterday I rode and made a couple of stops and when I continued to ride I noticed my speedo quit working and then it started working again for about 30 seconds and quit again. All my dash lights and odometers work. I've been told that I should check the the plug connections under the tank. I've also been told to check the speedo sensor. Can anyone tell me which one it probably is and if it's the speedo sensor how often do they go bad?