my car has had this weird idle for a few days now. not sure to what it could be.

it revs -/+ 500 rpms when it's idle.

it's weird. if i rev it 'till about 4 or something then gently bring it down, it will usually go away until i rev it hard / start going somewhere. then when i idle again, it's back. it's been cold and warm latly, and it never seemed to matter.

my lights dim out a little bit too. but my car has always done that. (figured it was the underdrive pulleys or something.

thanks in advance for the advice.