check ur throttle cable make sure that its tight and not too lose (supposed to have a little slack, but should be smooth maybe use wd40 if necessary) clean the butterfly valve on ur intake manifold with a gunk degreaser it might be sticking. but with a 99 u shouldnt be having problems like this yet, make sure u do ur regular maintance ie spark plugs oil changes on time, if u havent done one at all u might want to do a transmission flush get all new fluid in there might make things smoother but once again a newer car like that u shouldnt have these problems, its probably jsut not feeling like a new car when its old like my 88 then u'll know cars just arent perfectly smooth all the time