Help with Ibanez DM1000 digital delay

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I have an Ibanez DM1000 rackmount delay that intermittently makes modem-like sounds on the delays. The problem seems worse with hotter signal levels going into the unit. I'm a digital n00b and don't quite know where to begin.

Here is a link to a pdf of the schematic if anyone can help:

http://www.mediafire.com/?05nmajnmn4z

Thanks.