Car dies in park or neutral need help please?

I have a 2000 hyundai elantra gls. It has 87000 when i start it i have to pump the gas sometime and i shouldn't need to since it is fuel injected. When i place it in gear and it idles it does fine idles at about a grand and then when i take my foot off the break and hit the gas it drops rpm to about 5-600 and then takes off fine. When cruising and i let my foot off the gas it steadily drops rpm to almost a grand or so. its a gradually but not like my other car. this car acts like there is not more power in the car. if i hit the gas it jumps back up no problem. But yesterday it just started this. I was talking to my buddy had the car in drive foot on the brake no problem. pleased it in park it sat for a min and the bounced rpms between 400 and 1300 and then dies after a couple of time and then other times it will idle fine and i noticed it does it also if i place it into neutral. not sure if these all are tied together but thought i would put everything on here and hope for the best.