What you are describing very likely is a servo problem. Are you seeing any twitching of the probe leg when it should be sitting still?
Are you using a blue TowerPro servo from eBay? Those are of horrible quality.

The EMax-ES08A servos seem to give very good and consistent results. And they are not very expensive.

http://www.ebay.com/itm/EMax-ES08A-I...QAAOSwGtRX01j-