[Ginto8]

Although I agree that your issues are more likely mechanical, I have seen some robots that have drift problems that are not extremely severe (and thus difficult to properly diagnose), so there is some application for using encoders to even out drive. The basic technique is to use a feedback controller (eg. PID) to control the difference between the encoders. eg., if you're using (rightEnc-leftEnc), you'll control the value to 0 to move straight, some positive value to curve left, and some negative value to curve right.