That's usually predictable - it happens when you change directions, and doesn't accumulate or occur randomly. I'd suspect the "Sure-step" motors you used instead. What you're probably experiencing is step loss, which can cause problems like you're noticing. Even if the holding torque on the motors is sufficient, they may want more power than they're being given by the Sherline CNC system, or they may have too high an induction factor to respond swiftly enough. If you've tried lowering velocity and accelleration without finding values that give predictable results, then you might look for motors that are better matched to the controller, or a controller that's more compatible with your motors.
Oil is usually better to use on these machines than grease, which tends to collect dust and form a clay-like substance that gums up the works, but I doubt that's your problem at this point.
ComputerSculpture.com — Home Page for Discount Hardware & Software