And are you serious about the gold and silver and coins?
Because I'm not.
I don't have any diamonds to trade for mics.
I don't think the wife would approve.
But it's something to think about.
Copy the wav files from the SD card from the Zoom into your computer and import them into Reaper. You can also copy them to the computer directly via the USB cable.
I had an MXL 2001 mic that had the same exact problem a few years ago....it wasn't a humidity type of issue that I could tell...at least, doing the standard "drying" techniques , did not correct it. I could not find anything obviously wrong inside with connectors, wiring, or the circuit boards...