I bought a 9300 and had the exact same problem. After reading online that the Trackpad of the Curve 3G is crap, I returned it and got a 9700, which is very precise
The trackpad on the 9300 is a bit elevated / curved, as opposed to the flat out Trackpad on the 9700 and the 8520.
This slight curve is the problem where it tends to make your finger stick at a certain point and making the motion not so smooth.