I've had the same problem with the Pioneer 118BLK running 10.5.8. In fact, I've sent it back to Pioneer twice for repair. I've paid as much in shipping it back to Pioneer as I paid for the unit in the first place.
Third time is not a charm as it continues to freeze my computer when I have a DVD installed and I try to access a file from that DVD. Just got off the phone with Pioneer again. They are replacing the unit with a new one (finally), but I have a feeling that the issue is a compatibility problem with 10.5.8. I've run Disk Utility, Disk Warrior, zapped PRAM, stood on my head and counted to 10, but nothing works. My G5 runs like a champ when this unit is not installed. I'll report back when I have the new unit installed, but I'm not optimistic. I could be reduced to finding a legacy Superdrive on eBay.