You could have fuel delivery problems caused by several things. First check all your fuses, including the 25-amp blade-style fuse under the hood between the battery and distributor. Then check the fuel pump relay and replace or re-solder it for goodness. Problems with these things typically result in no-starts or hard starts, though, which is not your symptom.

Next suspect area is the fuel pressure regulator. With the engine off, pull the vacuum line and sniff it. If you get a strong fuel odor, replace it. If not, replace the vacuum line, start the car, then remove the vac line and plug it with your finger - there should be a change in idle speed. If not, replace the fpr.

While you're under the hood, inspect the intake air snorkel - the long hose between the AMM and the throttle body. These crack and get holes worn in them, typically on the underside where you can't easily check. A big enough crack will definiteley make you car run poorly.

Last thought has to do with the fuel pickup tube. If your car runs fine on a full tank but poorly with half a tank or less, the flexible pickup tube in the tank has failed and you'll have to pull the pickup and in-tank pump to replace. Lots has been written here to describe that job.

These are just tip-of-the-iceberg thoughts. Check them out and post back what you've found.