Long ago ,if I remember correctly I had this issue with a long gone vehicle & noticed that if the load on the elec. sys was changed ( brake pedal) it would drop to zero. So perhaps it was related to power side. Ran a line direct to battery & problem solved. Box current draw is minimal so wasn't concerned about powering all the time. If that should be of concern use a relay for power when key is on.
Worth trying ??
