Stellar,
I'm an industrial electronics technician, and right away, a problem jumped out at me... you said you have your daisychain grounded at two points on the iPac, right?
This is a potentially bad thing. I know that if you daisychain grounds on relays together in an assembly line control panel, they tell you to NEVER ground the daisychain at more than one point, because it creates something called "dual ground plane potential shift" or some technical BS like that... basically, it just means that the amount of power travelling through the groundS varies depending on your switch's position in the daisychain, and the amount of resistance of wire in each direction... it's confusing, and I didn't completely understand everything the teacher at the factory I was training at was saying... but he got across the point to me that you're not supposed to double-ground anything wired in parallel OR series.
Hope this helps --dave