Digital Glitch using Adat, problem 1 with recording system Options

[Cpt Horse]

I've Being using (for quite some time now, about 4years)

Mac G3 266MHz 96meg ram
OS 7.6
Korg 1212 i/o audio card
Alesis Adat
Cubase v3.5
all running at 48kHz sample rate
the audio card is the master time clock source
the adat sync to the audio card

i use the Adat as a digital interface with cubase
eg. 8 channels analogue directly in to the adat then routed via the optic cable to the korg 1212 i/o.This gives me 12 channels of simutanous recording with the analogue and digital inputs on the audio card

this system is rock solid and it has never failed me as a basic recording tool.
and is used in a profesional studio.

But with one little problem thats now starting to bother me.
I get what apears as a random digital glitch while recording,
its a 'tick' noise, it will peak above 0db and only on any one channel of the incoming 8 from the adat. it might happen once every 5 to 10 min or once a day. in the pass i would just zoom in on the recorded data and edit it out.  

what i want to know is if anybody else has a simular system as the one above with the same problem or if any one might know what might be coursing it?

thanks.
Cpt Horse