Your ilde should be around 1200 rpm for the first few minutes while cold until the oxygem sensors warm up and the engine goes into closed loop. At that point the idle should be down around 750 rpm. I would check for a restricted exhaust. You can attach a vacuum gauge to something on the intake and watch the gauge, if it rises to around 17in and then starts to slowly drop, you probably have a restriction in your exhuast. You can get under the car and detach the catalysts and retest. If the engine vacuum holds at 17in and doest drop then you have isolated the issue. Most likely your catalysts are getting plugged from age.