First, you must find out, where this deviation comes from! So, is it noise? If so, is it Johnson noise from the load cell? Or is it RF noise entering your circuit, because you have omittted even the least amount of HF filtering? Or is it hum, which can be shielded against? Or is it common mode noise, because you have connected certain boards in a wrong way? Or is it instability, means HF oscillations, because you haven't filtered enough certain signals or because you have omitted the obligatory ground plane? Or is it mains interference, because you have chosen an improper grounding (connection to safety earth?)

Or is it something much more nasty, namely repeatability error, means hysteresis?

So, I have several dozens of possible reasons in my mind, why your reading isn't constant. And instead of analyzing where the varying reading comes from, your only idea is to use an external OPamp???

Kai