Problem with 2000 Chevy Impala LS?

[nikeman2k2]

The RPM's throddle up and down by itself and they car dies at stop signs. This does not happen everytime I drive only sometimes. I know I need to fix it but I dont know what it is. I looked it up on the internet they said replace some small piece and I did it, it was only $20 dollars( i cant remeber what the piece was called) but that did not fix it. My car has 150,000 miles and I have no idea if anything has been tuned up on the car I bought it used 2 years ago. Yes, I know i need a tune up too!!