97 Honda Prelude type sh idle and check engine codes

I have a 97 honda prelude sh with 95000 miles and i am having a couple of problems. first problem is when i start my car when cold my idle will jump up and down slowly until my car starts too warm up then it stops. then sometimes after driving for like 30 mins it will start it again. this doesn't happend all the time but part of the time. i have checked for vaccum leaks and found none. i have replaced the idle air control valve and that didn't fix it either. so what else could it be. i also have two check engine codes which are p0450 , p0453, which i think has something to do with the fuel. but i put a new gas cap on it and that didn't fix it. if anybody could be of some help just let me know of what i can do. thanks a bunch.