Just out of curiosity, you mention using an electronic scale. Do you, or have you by chance, moved it recently, and if so do you have a fluorescent light on or close to your bench?
Reason I ask is, I have two different electronic scales, and have found with both of them, if I get them within 3-4 feet of any fluorescent lighting fixture, it will drive the readings nuts. The higher frequencies generated by the transformer seem to do weird things to the scales internals. I can zero them and they will stay for a minute or so then start to wander around while weighing. I can set a bullet for example on the pan, and hold my hand between he fixture and the scale and literally change the readings depending on where my hand is. I usually only use them for weighing out bullets, or brass, but I have learned to steer clear of the fluorescents.
This might be what your experiencing if you have recently installed or have moved your scale a bit closer to a similar fixture.
Just a thought.
Mike / TX