450 SLC, engine surging at idle

Haven't driven my '73 450 SLC for a while, so I took it out for a spin the other day. Its developed a problem where the engine idle will jump from 500 RPM (normal)
to 1500+ RPM, then down again. This repeats constantly. Aargh....
Does anyone have any idea what might cause this? I did try adjusting the idle, but that did not make any difference.
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