Speaker cables are not sheilded. They carry a higher voltage which is not so susceptible to interference. Thus no sheilding. But the main reason is a sheilded cable can overheat, short, and fry your amp, catch on fire, all kinds of fun stuff.
That said, you can use a guitar cable for a quick test at bedroom levels, but I would not leave it like that.
Hard to say for sure why your levels are dropping. Could be anything from a bad solder joint to a bad power supply. Try changing out one peice of equipment at a time. First thing I'd do is plug in a different guitar and see what happens.
...so ever since then, I always hang on to the buckle.
