Good to see Simon got you on the right track.

FWIW, I typically don't put any shielding tape in the jack cavities of strats or teles. IMO the benefit is negligible and the chance of having a short are increased. I'll also point out that applying the copper tape to the metal jack plate is unnecessary as the plate itself is conductive.

There are alternative ways of dealing with an unshielded jack cavity, such as using coaxial cable as the output leads from the pots/switch to the jack, or twisting the positive and negative wire together in a helix fashion then applying heat shrink tubing over the pair (literally creating a twisted pair).