Misfire in cylinder six

The car is located in South Florida and problem happens occasionally and only for a few seconds.
After the car has been driven about five miles and does not reappear for the rest of the day and generally not for several days or even weeks.
Scan tools indicates misfire in number six only.
The wires, plugs, cap and rotor all check good.

Even though it is an older car with high mileage this is the only problem, everything else works.
The van is my work vehicle and driven daily. I am the only owner and have followed a strict maintenance schedule

The local garage though it me dampness since it only happens in the morning. They tried spraying the ignition system with water but it ran perfectly. It generally hot, and humid here

Dave 429