Viewport in (Cycles) Render mode flashes continuously: "path tracing sample" msg in vuport

I’m attempting to use Cycles/Render view in the 3d View but it flashes continuously, the mesh is heavily pixilated, and the line “Path Tracing Sample 1/128” is flashing in the status msg in the upper left.

After I did an F12 render it stopped for a bit (maybe), but soon resumed.

EDIT: more info: [redacted] +AND only in Edit mode.

What’s up?