hmmm... there is no reason why fw800 wouldn't work with an adapter-albeit at the slower 400 speed... You say your iPod came with a fw800 adapter? are you sure that isn't just a 6pin-4pin adapter? both of those are for fw400 ports. fw800 ports have 9pin ports and you would need that adapter.
This is what you are looking for. I would suggest getting a regular 9pin-6pin adapter so you can use any cable... but you may get more use out of an additional iPod cable.
As for the usb problem... you say it has worked in the past, what have you changed in the interim? iTunes 4.7.1? 10.3.7? Maybe some new hardware?