Spark plug wires

I have a 2001 Kia Rio. Two years ago, I began noticing that the car was misfiring upon acceleration in the lower gears. The first mechanic said that I had a leaky gasket and that I could use new spark plugs. I only replaced the spark plugs since it was all I could afford, but it only ran slightly better and only for a very short time. I decided to bite the bullet and try to have the gasket replaced, but I took the car to another mechanic. The second mechanic said it probably wasn't the gasket, just the spark plug wires. So, I replaced the wires and it ran much better for about a year. Then, the old problem came back. Without consulting a mechanic, I replaced the wires again and the car ran fine for 9 months. The car is misfiring again and I am tired of buying wires, especially since I'm getting progressively less life out of them. The car is at a local dealer and I have little confidence that they can fix the root cause. They have replaced the plugs, wires (again), and the O2 sensor and it is not even better now. I suggested replacing the ignition coil, since the second mechanic mentioned it as a possible cause, but the dealer has assured me that it is fine. The dealer now suggests replacing the knock sensor, but I suspect they are just guessing at this point. Has anyone had a similar problem or know how it could be fixed. I'm an engineer, but know almost nothing about cars.