96 Trans AM Running Really Rich

Hello All! I have a 96 Trans Am that only has 80K on it. For the last year it has been getting progressively worse. It is running really rich and I am getting terrible mileage. From day 1 I have been running it with premium gas. I have since cut down to plain unleaded, just due to the fact that it doesn't make much difference because it is running so bad, so I figured I would save a few $$. The car is due for a tune up, but I was told the spark plugs I have are rated for 100K. I did change all the wires about 6 months ago, because one was jumping. I haven't worked on a car with as much electronic stuff as this one and am not sure what I am even looking for. I understand that if I take it down they can hook it up to the computer and get codes, but I am looking for some sort of diagnosis prior to that visit (especially wondering how much it is going to cost).

Thank You

hey sas11kid this is the grand am post, someone will move this over for ya.