Rmdwpn

Hi guys I haven't been Able to find it on here so I thought I'd just ask hoping someone could help me, I have a Lexus ls400 and it has an issue with the idle it always pulsates from about 1000 rpm to approx 2200 rpm it still drives ok but idle high in gear and revs it tits off in park, it's annoying and embarrassing, anyone know what it could be, all suggestion greatly appreciated Thanks in advance