My 93 Chevy Lumina 3.1 Euro Sport is having a lot of trouble

My Lumina's a 93 Euro Sport 3.1 with about 130,000 miles on it. When I purchased the car it ran great, but lately it's been having a lot of trouble doing really anything. It started out where it would just have a lot more trouble starting than usual. It took a little longer and sometimes more than one attempt. That was never a problem before. Then the RPMs started bouncing up and down while driving. They'd go from about 700 to 1200, up and down rather quickly. Sometimes they'd drop a little too low and my car would just die. Also sometimes it seems to just die without reason while I'm driving. As of two days ago, another problem occurred. I got onto the highway to head into town, and that was the last time I hit the gas. The engine was idling so high that it was pushing my car along at 55mph without me even hitting the gas. It did that the entire 15-20 minute drive into town. If I had to come to a complete stop at a traffic light, the engine fought me trying to stop, and then the RPMs would drop way down to about 500 and bounce up and down really roughly and my car would tug like it was trying to take off. I've talked to a lot of different mechanics and other people with knowledge of cars and no one seems to know what's wrong. Can you please help?

Note: I've recently replaced the fuel filter, air filter, spark plugs, spark plug wires, oil filter, oil, and the idle air control. None of this helped the problem in the least bit.