New 99 dakota

I just got a new 99 dakota sport, clubcab with the V8 318 magnum in it. When i sit idle the rpms fluctuate, the engin rev's up and down. I wouldn't mind that much but when i put it in gear it jerks. It sits around 1000 and goes up to sometimes 1400 rpms. sometimes i can put it in neutral and get it to sit down, then put it in park and it wont have any problems until i tap the gas. Any suggestions?