Tremendous RF problem -- phono stage is a radio

Rives...For many years I used shielded 300 ohm twinlead for FM signal. I am in a weak signal area, and noise from autos on the nearby road was a problem. It worked very well, but the surprising thing was that the shield was, by design, not tied to ground at either end. It was floating.

The various suggestions that have been made are worth a try. However, let me note that low impedance/high current circuits, which means a MC pickup and solid state amplification, are less likely to have your problem.