Yielded terminals in one of the plugs in the back of the ICU. Caused by vibration over time. Also check the ground studs on the firewall down by the pedals. Very common problem with that vintage. I have repaired this several times.
Third option is one of your wiring harness containing the data link is rubbed through at some point.
And last but certainly not least, and the least common. The Am/FM causing the data link to drop. Simple test is unplug the radio and see if it comes back...this is the rarest of the 4.