Yep, mine does the same thing and since my tub was replaced last year, this is actually the third spa monitor system. The old one also would also go into the anoying false alarm mode and my dealer replaced it. Then I got a new one with my new tub. What I have discovered is that you will have to move the location of your monitor...it's really that simple. I have no idea of why the signal from the tub would "move", but that is the solution that I found works.
My new monitor was mounted on the wall of my mudroom which is about 7 or 8 feet from the front of the tub and to the right about 4 feet. All of a sudden (after a year) I was getting the false alarms! And, like you, I wanted to toss that darn thing INTO the tub!! I have now moved the monitor right inside the door. Which is about 5 feet to the right of the front of the tub.
I am able to control everything in the new location, but the alarm hasn't gone off. This is the exact location that my other monitor was when it kept going into false alarm mode!!
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I will tell you that I explored everything that I could think of..my wireless phones, remote controls (for everything from the garage door openers to TV remotes). I have a real problem with things that I can't figure out, but on this one, I gave up and just moved the darn remote.
If you take the batteries out, you have nothing. ..Is it still under warranty?
Let me know how it works out for you!