[werk_ag]

OK, I'll keep an eye on it tonight as the sun goes down.

Last night after the sun went below the horizon the reading dropped to 0, and then increased a few points. Then for the next few hours the reading fluctuated a few points up from 0.

If there is a problem with this sensor the guy said he would exchange it, if he doesn't sell them all before I figure out if there is a problem or not.

The sensor seems to be quite good, those spurious reads at night surely are not caused by the sensor.
They can be also happens due to a presence of a very small voltage at the ADC0 input, 1 or 2 mV are enough.
If I was you I did not worry much about it. The filtering you are using is effective and doesn't affect the overall readings, it just cause that readings start at 4 to 6 W/m2