Thanks for expanding on the problem. Have you talked with anyone who has been is the same situation and not had the problem? Unless this is a design defect of your system (Makes a great sign off : "Design characteristic of airplane") I would inspect the HSI and related shielding of the wiring, to see if it is actually still connected as originally designed. Shielding doesn't get very much respect or attention in the field, but it can lead to write-ups such as this. Shielding is designed to keep your wiring from acting as an antenna and prevent interference and cross-talk between systems.
This could also be a 1960's system suffering from 1990's problems.