Instrumentation: Measurement Noise
Try the x1 probe setting and 20MHz BW limit and AC-coupled and you should see an improvement. 

Remember, an oscilloscope will have a very high sample rate (1GS/s) but quite coarse voltage resolution- in the case of your 'scope, only 8 bits vertical resolution (1 part in 256). 

If you want to see precision slow-moving signals you need something more optimized for that application such as a data acquisition system or a voltmeter.