As always, thank you for the helpful tips. Starting from easiest to diagnose and fix and working from there, I repositioned some of the wires and now it all works fine.When the no signal occurs, apply 12v to the D2 yellow line. Does it's fan turn on? If so the D2 ecu is at least nominally working.
You have three possible causes. The EST is faulty, the wiring is faulty, or the D2 ecu is faulty. I suggest cleaning and checking all the connectors. Reposition them to changr the stress on the wires. Put some dielectric grease on the contacts.
Running a bypass jumper directly from the D2 main harness to the EST, bypassing the other wiring may be a useful diagnostic tool.