The Service Dept at Horton Isuzu actualy found where the aftermarket weathershade, properly installed, was causing a slight-enough binding of the driver's side window to cause an offset.

The relocated the front-end of the weathershade, so moisture could seep in, but at least the binding effect has ceased.

I'll probably remove the weathershade and try shaving off a millimeter or two of the thickness in the plastic tab that sits along the outside ofn the channel. Then reintall according to the manufacturer's instructions.