To me, your description of the problem sounds like a bad output-jack in your guitar. The easiest wayto test this theory, is to get another guitar and plug it into the amp - if the problem ceases, then your guitar is definitely at fault.

Of course, as ManXcat suggests, there could be other internal problems with your guitar's wiring, as well as possible problems with your connector-cable - however, I'd still guess that your guitar's output jack needs replacement. It's an easy repair, and the replacement Swithchcraft #11 output jack only costs a couple of bucks. if you don't have decent soldering-chops, I recommend taking your guitar to a local 'pro' - they shouldn't charge you more than $20 to $40 to replace the jack (unless your guitar happens to have no access-panel, to get to the jack)...

Stephen White