Semi-acoustic guitar, a lot of background hum when played through amp

On my new electro-acoustic, there is a signifcant amount of hum when played through an amp. If I lightly press down on the jack on the guitar (or the end metal bit on my leads), however, the hum entirely disappears. Nothing seems to be loose. Is the problem with the gold-plated jack on the guitar itself? How do I permanently get rid of this hum?

I've tried this with both my PC speakers and amp, and two different leads btw.

Didn't know which forum to post this in, but hopefully the Pit can help me on this one.

Last edited by Jake™ at Nov 27, 2008,
I play a semi sometimes but i dont have that problem, however i do with my Strat so what I did was Tape some black Gaffa tape to it (very small peices) and it sorta weakns the jacks electrical signal (I think, but it worked anyways)