[danner]

I will try to give some simple figures that may help to understand to what we are dealing:

Supposing that R2 have a value of 4900 Ohm, the gain of the OP is around 3, this mean that by each mV on the input the output will be 3 mV.

So, if the pyranometer (or any interference) presents a voltage of just 0.33mV in the OP input, that is enough to produce 1mV at the OP output, which by turn will produce a reading of more than 1W/m2.

Out of curiosity, is that change in gain linear?

If the gain of the OP-amp is "3" with the pot at 4.9k ohms, what was the gain with the pot set for 4.7k ohms or after I changed it to 4.81k ohms?