Unstable voltage output for strain gauge application
You are seeing noise. Exactly where the noise is coming from depends on your circuit layout and what the source of the noise is. (Sorry, bad definition but you know what I mean I hope). It may be difficult, or even impossible to eliminate, depending. Diagnosing it could be harder still, without trial and error or suitably advanced equipment and technique.

Instead, try taking a number of values and calculating the average. Up to a point, the more samples you take, the more accurate the reading. I had a similar problem where I took 10,000 samples each time I wanted to get a good reading, and took the average. This change increased the accuracy by a factor of about 1000! However this will not necessarily solve all your problems. The final accuracy will depend, for example, on the distribution of the noise that is present.

In your code, remember to convert integers to floating point variables before these calculations.