Ok, I think I may have isolated the problem.

There is a single circuit in the house that dims when a high load turns
on anywhere in the house. I managed to cycle the boiler and it caused
the lights to dim. I then powered on a table saw and it also caused
lights on the suspect circuit to dim.

How do I troubleshoot this circuit?

Should I start in the panel and work backwards?

The circuit has aluminum wires that are spliced to copper with the
special purple wire nuts used for AL-CU connections.

Michael