[ossiblue]

Sounds like the vintage radiois affected by the ignition components--CDI and spark wiring. The early cars had shielded ignition wires, does yours still have them? Also, there were some noise suppressors often located in the engine fuse panel that might have been removed. If you post a picture of your engine compartment and electrical panel (in engine compartment) we can see if the devices are present.

I believe you can buy suppressors to install along your radio wiring, but real experts will have to verify how it is done.