If it's jumping from one gear to the next, I'd be wondering about the condition of the cogs and/or chain. Are both of these components used? If so, they are probably worn out, and need replacement.
Otherwise, I'd keep adjusting the derailleur. Perhaps the derailleur is not compatible with the shifters-this can happen when you mix-and-match old and new components. Chances are, a 6-cog derailleur is not compatible with newer stuff.