Prusa i3 PLA fails to stick to first layer on curves
It could very well be that your print speed is too high (starting from the 2nd layer) for the motors to keep up comfortably. Try reducing the print speed to perhaps 75-80% of what you are using to see if it addresses the issue. You should also check you model to make sure there are errors in the STL. Sometimes such mesh errors can cause funny things to happen. Do you see the irregularities in the print preview at all?