low idle?

my car used to idle at about 500 rpm , now i think it getting a little lower. i sat and watched it yesterday for a while, it would get low then come up a bit after a while, but not over 500 is this a sign of a problem . if so can i have some insight into what it might be.

for one the normal idle should be around 750RPM. If it's any lower than it might have to do with the distributor. If not....just turn up your idle by adjusting the know by the intake manifold.