My car idle's hard WHY???>>.....

Sometimes when i turn my car on it will idle hard like your stepping on the gas it is loud I can even switch it from park to drive and it still does it and then it will go away when it wants....Ok I can also be driving and it will do it when i stop at a light I can figure it out....can anyone give suggestions ....I have a honda accord 2001 it has 50,500 miles...i got it used with 38,000 miles and i have changed the oil every 3,000 and i got the trans fluid changed and i got a new battery i got a new header...and cold air and exhaust i just don't know why it is idling hard sometimes...thank you in advance....

how high does it go ?

does it stop once the car is warmed up?

see it idles hard but it doesn't show it on the rev line..it stays at like half of 1 like 500 or so....it stops maybe cause it gets warm i'm not sure though if that is why it idles hard.....i mean it won't last longer than thirty seconds usually i just shut the car off and then start it up again and it won't idle hard so i don't think it's a cold issue i live in texas hahaha ok thx so far anyone else?....

i bet its jsut a cold start thing