Presonus Firebox, problems with recording 4 tracks Options

[manuel Loos]

hi there,

i got a problem recording 4 tracks simultaneously on CUBASE SE and Logic express with my FIREBOX.
for some minutes (sometimes up to 20) it works fine, but then the recording stops, my computer tells me
a) ASIO is out of Sync! (Cubase SE)
b) system overload, disk too slow ..... (Logic)

i use a ibook G4, 1,2 MHz, 768 Mbyte RAM.
the recordings are without any other trracks played back and no plugIns are used.

does anybody know what this could be? is it the "slow" processor i am using?

thank you for an answer!!
cheers