Mackie onyx 400f stops working in OS X 10.5.6

I recently got a Mac Pro quad-core. I am using the firewire cable adaptor 800->400 so that I can use my 400f audio interface and everything seems to be working fine, but at some point during playback in logic the onyx will just stop making sound. The activity of all I/O within logic or the onyx's interface or even the onyx itself is shown as normal (dynamic movement means it's playing back something) but nothing comes out the speakers. This is only corrected if I "restart" the device, which is no proper use.

Anyone?
Ps. I've been using this card for ages, so it's not something I do wrong. It's the **** firewire, but anyone come up with a solution plz!