Gads, many things come to mind. Have you looked at the throttle cable "seat" at the firewall? Those plastic clips can be an issue. Is it stick or auto trans? The most significant improvement in my idle quality (after repairing dozens of vacuum leaks, basic tuning proceedures, self diag ECM,etc) was a clean and adjust of the throttle switch (TPS on auto trans units). If you have a manual box, I suggest you read Troopersphere's well written article on tearing down and adjusting this switch, my ECM never knew idle until I did that job. The old brute has manners now.
Check a few things out and post results here, perhaps we can nail 'er down.