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weird question about amplifiers.

Category: Hardware
Product Number: AD620

I have a strange problem. 

I have an loadcell amplification circuit that uses AD620, and OP177GPZ. I have been using that circuit for years. Recently I noticed something.

Some of the boards if I touch OP177GPZ with my hand output starts to shift. It does not oscillate. Output gradually and slowly shifts. But some boards there is no such a problem.

Also when I blow hot air with my mouth some boards output shifts much more than on some boards. When I replace components like capacitors and OP177 or AD620 it does not change at all. It is like something in the PCB. But PCB is same. Can it be something related to assembly process?

Does any one has an idea?