I'd look at the power washing as a possible culprit. Finding the offending droplets of water might be a challenge if that's what it is. I would look all around the ignition wires and spark plugs for condensation and try to dry it with a rag or even a blow dryer. I would also try to run the engine hot for about 10 or 15 minutes to see if you can dry it up that way.

I'm always very careful now whenever I spray my engines down as I've experienced a few weird problems due to sensors and wires getting wet. It's best to cover those parts with a plastic bag or something (making sure the engine compartment is completely cold!).