Use a voltmeter on the signal pins to see if the supply was in spec. More at http://pinouts.ru/Slots/ieee1394_pinout.shtml See those pins 1 and 2? Lotsa power.
A while ago, I had a problem on my G4 where several FireWire devices failed in quick succession - a hard drive, an Iomega Peerless and the the connection from my Sony Camcorder.
My idea that the G4 could be the source of the problem was reinforced when smoke came pouring out of the base of the Peerless! Following that, I was charged good money to change the G4 motherboard, which seemed like overkill to me.
1. Has anyone else experienced this problem?
2. Is there any way a non-professional can check the FireWire connections on a computer to make sure they are working as they should?