Check in the Analyzer menu, there is a reading of what the BDI voltage is at A2.4.26. If it is varying a lot and not steady then take off the access plate next to the footpedals and check the fuses for corrosion, FU6 is the culprit and is shown as LIGHTS/FAN but also supplies BDI voltage to Access 4. You can check the voltage at Access 4 CA412-3 also. Had this exact issue a few months ago with the symptoms you describe, took a while to locate it.
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