"Pulsing" voltage, RPM drop with load.

Thanks to all. I'll get some readings with my fluke today.
Just to be clear. The cabin numbers I stated were from my volt gauge which IIRC is connected to my cigg lighter.
I have scraped paint off and used dielectric grease and star washer on firewall mounted VR and Ign module plus have added
ground wires to each. I do run a small amp but have a heavy gauge feed wire.
I hesitate to mess with the bulkhead connector......yet. The car is originally from AZ so everything is brittle. I've disconnected it once and broke a tab off. My plan is to clean connectors somehow and pack with grease but only want to do once and I am planning an engine swap this winter. Maybe I'm being paranoid.