Lots of people have the symptoms you describe, wether, or not, their engine has been power washed. Usually the symptoms you describe involve, almost exclusively, the starter system.

That system includes the battery, the starter, the ignition switch, the tranny safety switch and all cables.

Next time it doesnt start, try wiggling the gear shifter while turning the key. If it starts, theres a problem with the gear selector safety switch(usually adjusting it will solve the problem).

If that doesnt solve the problem you need to check the battery and cables. You could have corrosion on the battery terminal(take them off, clean the posts and inside the cable connector), or a loose connection at the starter.

You could also have an intermittent bad ignition switch. I check for that problem with a volt ohm meter. Disconnecting the ignition wire from the starter terminal and mesuring for battery voltage when the key is turned to start. If you get battery voltage and it stiil doesnt turn over, that indicates a problem in the starter, or its main battery connection. If you dont get 12 volts when the switch is turned, its probably the switch.

You can also have corrosion inside the battery cables. With a volt ohm meter you should get close to 0 resistence from end to end.

also check the engine ground strap.

Batteries can also have internal shorts which could cause intermittant no starts.

If you arent mechanical take it to a shop. it doesnt sound ,to me, like a serious problem. Once again, the power wash may not be related.

Good luck