Watched a buddy try the same thing on a long fence line last summer and the laser walked on him bad once the sun got high. He ended up stringing it after lunch and finished fine, just lost a few hours like you did. Funny how the old cheap way still wins when the sun is beating down.

Tried that same Harbor Freight bath trick on a set of Epson carts about two years ago and I'm going to be straight with you, it isn't the miracle it looks like at first. Yeah, the nozzle check comes back perfect, but what you're really doing is blasting ink and junk back up into the cart body and the foam inside, and that stuff has to go somewhere. On the sealed carts with the sponge, mine printed great for about three weeks and then started dropping whole colors mid page, way worse than the streaks I started with. The cheap units also heat up more than you'd expect, and heat plus the wrong cleaning solution can swell the seals or dry out the print head if any fluid creeps past the plate. So for a dental office where those forms have to go out every single morning, I'd say the ultrasonic is a short term save, not a fix, and you might be trading a slow clog for a sudden dead cart right when they need it. If you keep doing it, use plain distilled water or a real cart cleaning fluid, keep it under a minute, and dry the contacts before you drop them back in.