Since you changed the discs and the cables and still the problem remains, there must be something wrong with the controler. The only thing you changed was the fsb. So, the conclusion that your controler doesn't like the higher fsb is quite likely. If I am not mistaking it, the controler runs at 33mhz (as all pci-slots). That means there is a multiplier of 1/2 for 66mhz fsb. Now you increased it to 100 which means 50mhz for the controler. That's an increase of more than 50%. A very high rate even for hardcore overclockers.. A wonder your optical disc still runs fine, but maybe they are a bit more robust due to their lower speed compared to harddiscs?!?
Beside that, I remember someone else who successfully overclocked his ibook but the benchmarks were quite disappoiting. Sorry, but I think reverting it back would be the best idea.
