Getting here late, but I think you should measure voltage drops on all connections between the battery and the starter (and the B+ line to the fuse box too). Compare the voltages to what you measure at the big red wire on the back of the alternator. The 2000 model year cars are particularly prone to corrosion problems in the positive cable, and as others have noted, they get really hot.

The positive cable is a pain to replace if you route it the original way, where it passes through some kind of cable tray near the bottom of the engine compartment. I routed mine outside it, but covered it with wire loom covers. Some of the aftermarket replacement cables are very dodgy quality, but the Volvo ones are outrageously priced. You may be able to find a cable that is sufficiently long with a stud on it for the B+ connection, which has a 10 mm stud and nut on it.

Good luck!