sounds like it may be a faulty idle control valve under the air box cleaner,its a big round aluminum valve with 2 black rubber pipes going out of it,or could be a bad o2 sensor.remove the idle control valve clean itout with carb cleaner,also spray carb cleaner down the air sensor plate under the air box.this may help if there is a build up of carbon,also run a bottle of lucas fuel injector cleaner,i have good results from it.