Hi,
You are running an 424 PCI card connected to the 24i and a 828MKII connected to a firewire port?
I think you are having a clock problem between the 2 units.
You need to pick a "master clock", the 828MKII has word clock in and out, the 24i has a word clock in/out sharing the same connector. I would use the 24i as master, set the 828MKII clock to external and connect the "word clock in/out" of the 24i to the "word clock in" on the 828MKII.
Make sure to set the "clock source" of the 24i as "Internal" in the motu audio setup. Make sure to set the "clock source" of the 828MKII to "word clock in" in the motu audio setup.
Also, Did you ask MOTU if it is recommended or possible to run the 828MKII and the 424 with the 24i at the same time. I would ask them this question right off the bat, you maybe trying to do something that is not technically possible. I am not sure first hand, I only run a 828MKII with external converters on adat/lightpipe.
I hope this solves your problem.
Out,
Robert
P.S. I just thought I remember reading that some G5 computers had a PCI /firewire problem look up your machine and see if it is one of the G5's with problems.