2005 Deville had complete tuneup. It is missing every so often. What is going on

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Had fuel filter replaced and other stuff, like plugs. Could it be the wires? Not sure. It has 190,000 miles, well taken care of. Oil leaks below engine, cannot afford to have it fixed. Am disabled vet, this car has to last longer. Returning to show next Tuesday, any suggestions? The missing usual around 55-60 mph. If I floor it, it does not miss. At times up small hill, and other times on level ground. not all the time. Filled with premium gas it seem to help.