put yer shoes on....it was my solution when I was all of 9 and first got into electric.. I would go down to the basement on a hot summer day and get ready to practice..hang that guitar on my shoulder plug in and turn it on..all ready to tap my bare feet on the tile floor...and wham...would I get grabbed...sounds silly..but that's the first suggestion I offer..cause many of us like to play with our toes free...
the second watchout is that the kustoms are not grounded in today's sense..with a 3 prong..the old polarity switch allows you to sync up..and that's actually a good way to get away from the 60 cycle hum..sometimes...but yeah..it comes at the cost of a fried lip.. I always touch the strings of my guitar to the mic before grabbing metal myself..just to be sure my amps are all in harmony...
but for you to be catching it just touching the guitar tells me you're physically grounding out..and I can't imagine that happening unless you're plugged into two amps, or a pedal board in line, etc..or have your feet on the floor or an arm resting on a metal chair..hope you find it..and lets us all know what it was....