I am assuming you meant that you have a 4.5L engine. I have had a few of these cars. They are fairly picky about their fuel. You will notice that on the dashboard, next to the fuel gauge, it does say to only use premium gas. They were serious about that. So it is quite possible you just got some low octane gas, maybe even a little water in the tank.
Also, being an older vehicle, there are miles and miles of vacuum lines under the hood that have been drying out over time. You should probably open the hood and take a good look at the lines. If you see any that are visibly cracking, then you should replace them. Look at the images in the following article for some examples of bad vacuum lines: https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
If not that, then when was the last time that the car was tuned up? It may be time for new spark plugs, plug wires, distributor cap, and rotor to be replaced.
Thursday, June 15th, 2017 AT 8:41 PM