I have 2003 Focus with 79k miles on it and just purchased it a few weeks ago. At a cold start in the morning or when it's been sitting for 6 0r 7 hours it will idle rough until it it's been drivin for several miles. Sometimes sooner. The engine seems fine and the transmission shifts fine also. I have checked the oil and trans oil abd they are fine. I did check the Air filter and it does need changing, but haven't gotten it done yet. The local Walmart was out of them when I went. When it warms up its idles fine.

Does anybody have any ideas what it could be?