I have a 1994 lumina that did the same thing. After I put new plugs and wires in it stopped. Now as you said you tuned it here is what else I would check.
1. Make sure none of your wires got melted by the motor for it to fire there.
2. Check the throttle position sensor(TPS).
3. Make sure your distributor is on snuggly and all wires are connected firmly. You'd be surprised how often a problem like this is solved by simply tightening a connection.
4. As previously posted check and make sure your fuel pump is working properly. Connected good.
5. Check for a blown fuse.
6. Have you're control module checked. It is located under the distributor cap.(Not likely but you never know.)
7. Have your alternator checked. A loss of power could indicate a weakening alternator. Also check connections. If the are loose then at faster speeds the could lose connection and drain power. They can reconnect by sitting over time.
I hope this helps in even the slightest way. If not, sorry for wasting your time.
Tuesday, June 5th, 2007 AT 2:31 PM