Hi dclm
The typ. zero drift of AD620A is 0.3µV/°C. Gain 660 gives 200µV/°C. Then comes the output drift typ. 5µV/°C.
This 205µV/°C is amplified with 12 giving 2.5mV/°C. Over 40°C it is 100mV. Added to this the load cell will contribute with more drift. Do you have a preload on the load cell? E.g. using it between 8 to 8.8kg? Then the poor gain TC of the AD620A, 50ppm/°C will add a lot more drift. 16mV*50ppm/C is 0.8 µV/C, added to the 0.3µV/°C on the first row.
You can measure the drift of each unit and program a D/A-converter to drive the reference pin to balance the drift as a function of temperature or use another amp. A quick search of analog's Inamps led to AD8228. It can be set to 100V/V gain and then you have to increase the following gain from 12 to 79.
To get low gain drift, select among inamps using only internal resistors for gain selection.