I’m sure the board has dried by now… Usually once the board dries completely it will start working back to normal again (there have been many cases of brushed FCs getting wet or flat out soaked and after drying completely they work just fine again, however YMMV.) If you fly wet conditions a lot you may want to consider some water proofing techniques using a thin layer of silicon adhesive to “waterproof” electronics. Even then this method may not be 100% perfectly waterproofed, but still helps more than no waterproofing at all! )
Could you post a picture of the flight board in its current state?
When it drops out and loses power, does the power ever restore, or is it just completely dead on the ground?
When wiggling the power cords after it shuts down, does the power come back on, even momentarily?
How many different batteries do you have to test with, and what type of batteries are you using?
Are batteries fresh in the TX?
It is good that you tried to re solder the power leads and RX, it unfortunate that didn’t cure the issue…
Whenever you have the time, you should try to test it again…
I would fully charge up all batteries before test, and do test a flight. If it drops out of the sky again, check to see if it is fully shut off. If it is, wiggle the power cables to see if you see lights come back on (could be a bad/loose JST or micro JST connector that is losing contact?)
If wiggling around the wire doesn’t signal any issues, I wold try and test flight with a different battery. If the same thing happens again, then I would try to once again, re solder the power leads, using a LOT of flux to help the solder flow to form healthy joints. (if you don’t have flux paste, or a flux pen, I strongly suggest getting one, as a strong solder joint from battery to FC is crucial for maximum performance, and preventing brownouts…)
After re soldering the power leads AGAIN, I would do another test flight. If it still drops out of the sky, then to be honest I dont know where to go from there…but definitely let us know if its still happening or if it has been resolved. I’m sure someone may have a better solution!