I have a 1995 F-150 5.0L Auto 2wd. When the weather is rainy and damp it doesn't run very well. At about 45 MPH it starts knocking like it is about to die. Once it has been running awhile it gets better. Could this be an electrical problem (like something is getting wet)? Could this be an air or vacuum problem? The air filter is new, but it is about time for plugs and a new fuel filter. Any thoughts or suggestions would be greatly appreciated.

 

If this was my truck the first thing I would do is change the plug wires and cap/rotor.