I had the exact same problem with my LS400. I changed the throttle body sensor and took off the throttle body and plenuim attatched to it and found that i had MAJOR carbon build up covering the inside of it. Take carbourator cleanor or paint thinner and clean the carbon out of both of them. I haven't had the problem since. Try this and if it doesnt work then you might have to resort to an idle air control sensor, which will run you around $500. ouch. But doing this will most likely help alot and give you back your feul economy and precious horses you know youre not getting :)
