Im all too farmiliar with this lol.
Well as far as it idleing at 1000rpms, is completely normal till the thermostat closes, then it goes down to 750-800 rpm.
But the 1000 to 500 is kinda confusing..might be a vacuum leak somewhere, check all your hoses.
check or clean your afm as well,,might just be screwing up a bit. Yeah and IAC valve may just be playing up wen it's cold, check and clean it out
Hope that helps
cheers