I'm leaning now toward some sort of electromagnet interference because Dad has moved the computer from its original location that was somewhat isolated from other electrical circuits to an area under the dash where there are a lot of wires, new relays, etc.

I have always found the position of the electronic control unit (ECU) strange - under the right hand front seat. Apparently there was enough room under the dash, otherwise your father wouldn't have been able to put it there. Volvo probably experienced similar problems when working on pre-production 1800Es.

It could also have something do to do with ageing electronic components. Semiconductors don't really age, but electrolytic capacitors (elco) do. When that happens the capacity of the elco decreases. That could affect EM-immunity.

I expect the signals from the various sensors to be weak compared to the strong current that the high beams draw. When you switch on or off a high current, it can cause a spark at the contacts. A spark causes radiowaves that could garble the small signals to the ECU. Moving the ECU away from the spark will make it less susceptible to its radiation.

Also, a relay has a coil that activates the contacts through magnetism. When you switch the relay on, a small current is flowing through the coil causing a large enough magnetic field to activate contacts. You probably know that a coil is an inductive load. And you may also know that disconnecting a current from an inductive load almost always causes a spark (the coil and contact breakers in the ignition system essentially do the same thing!). It strikes me that the interference only occurs when you switch OFF the high beams...
If you want to keep the ECU under the dash, try relocating the high beam relay.