DC Offset from IC when PCB is flexed

Has anyone seen this? - We have a new smt pcb with 2 x LF356 (Op-amp) IC's on it (plus others).

When the pcb is flexed the O/P of each IC changes by �2mV DC. i.e. the board is acting like a strain gauge!

There is no oscilation and when I replace the smt IC's with DIP devices on 20mm legs, absolutly no problems.

Can anyone suggest a reason, or two?

TIA.

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