Replying to New deraileur or not?

i just bought a bike from a guy on craigslist--sick bike but it doesn't exactly have the components listed on craigslist (it's a specialized stumpjumper comp im guessing 05 or 06, whichever the m4 frame is from)  it said it would have juicy 5s and sram x9s with x7 shifters-- it has some avids that aren't hydros but are still pretty nice for me, and the drivetrain is mostly shimano lx deores with the xtr rear deraileur.  anyway, my first upgrade would be to get a new rear deraileur but its only problem is that it shifts back and forth between 3 and 4 or 4 and 5 area so im wondering if i need a new one.  I was thinking about just getting a tuneup and going with it becaues its still a sweet bike and I only paid 830 for it and i think i got a deal on it still.  what would you do?