3000 rev start up on impreza

hello please some advice had my impreza for 6 month car is a one onwer fmdsh gx auto 2003 . when cold car revs upto 3000revs and drops down to 1200 revs when placed into drive revs move up and down a little  when driving on cold revs seem to drive car upto about 25mph but once warm seems fine but slight hick in top gear at about 2000revs when you take your foot of the gas . i have had it into garage throttle body cleaned changed idle control valve had codes read old p420 showed but was cleared petrol cleaner and full service this is nice old car but dont want to spend fortune looking at this problem spoke to main dealer a the car had a previous problem when a different garage fitted stainles steel exhaust which has been removed to solve problem sorry if i have giving too much info just have have had nissan leaf for 5 year and cant remember how normal cars drive thanks