I think the key to this mystery is the mountainous horizon. Mountains produce very turbulent atmosphere with innumerable atmospheric convection cells, updrafts and the like. As seen from your location and the low elevation angle of Venus, it is easy for me to imagine that the brightness of Venus was affected by these unstable convection patterns. You should have notices a dramatic reduction in this flickering as Venus got higher up in the sky, or conversely, a rapid increase in activity as Venus sank lower down into the horizon.