Sometimes When I'm standing at a light the rpm dips to 500rpms or when I'm driving under 10mph and I stop it dips to 500rpm. I've had this problem since I bought the car.
like I told you in IM, I'd replace any vacuum hoses with holes in them. Hell, replace them all to be safe.
I'd also check your Idle Air Control Valve, which might need replacing or just a good cleaning with carb cleaner.
Another area where you coudl have a vacuum leak is your PCV valve. you can always get a new one for $5. Check all of your gaskets on your intake manifold (throttle body, pcv, etc.)