Guitar "Popping" Sound

Hi,
I was wondering if anyone could try and help me identify a problem I am having with my guitar.

I plug my guitar into my amp and it plays and works fine but every so often I get this "pop, pop, pop" sound. Its not loud, sometimes it's slow, sometimes it's fast. It goes away in a couple of seconds or when I touch the metal pin on the cord or the jack of the guitar only to come back a minute or two later.

The guitar is a '76 Fender Bronco.

It's not my amp because I have tried the guitar in two amps in two different outlets.

I have also tried using different cords.

I hope you guys can help.

Thanks.