Codes, P300, P301, P303 and P304 after driving several miles

For the fourth Tuesday in a row, my car had so many misfires I needed to reschedule my appointment. For the second time, I took it to my mechanic. The first time he told me to pull the plugs and check them. Yesterday they were closed at 4:00pm.
8/28 replaced spark plug tube seals.
9/04 replaced spark plugs (which were less than 14 months old).
9/11 replaced the distributor (leaked oil and the points were corroded) and cleaned the wires with electrical parts cleaner and Q-tips.
09/18 after driving about twenty miles the car misfired worse and worse. I cleaned the spark plugs and wires with soap and water, dried them with my compressor, and drove home without a problem.
How come the car drives fine for five miles, but then starts misfiring? It only misfires on the cylinders that had the oil leak!