Digital Camcorder-Bad Firewire Cable?

I’m a Mac user with 10 years of graphic design
experience (on computer) – little “IT” experience.
My question concerns a Sony TRV20 digital
camcorder which connects to a Mac G3 Firewire
port (6-pin, computer — 4-pin, camcorder). For
two months the Monster-cable I used worked fine
and the iMovie software recognized the camera,
but now, the software cannot see the camera. Is
there any way to “test” a firewire cable to see if
it is functioning properly (or not)? Would it be
better to buy the cable directly from Sony?