A friend as a Firewire port that's slowly going. As it gets progressively worse it starts to do some interesting things when you mount Firewire hard drives or cameras. One thing is this when you mount an HVX-200:
For me, the most interesting thing was that FCP would see the footage while the P2 card was mounted like this but there would be massive artifacts during playback. Plugging the exact same camera and card using the same Firewire cable on another Mac acted normally.
Out of curiosity I reset the PRAM, SMC and all that on the Mac without any of it helping.