I know what you are speaking of, I too have experienced this effect with various melodicas. (though I don’t remember specifically if my P37D has particularly exhibited this problem) – my suzuki melodion has for sure though.
So you get this situation where the internals of the instrument are saturated with condensation and water/salivations are clogging up the close tolerance of the reeds.
I have taken a few tacts to attenuate this problem.
1. as I learned is super important with the bass melodion, don’t bother with tonguing, it messes with the attack of the notes anyway, probably because of pressure propagation against the size of the large reeds. I too come from horn playing and have tried to use breath control for some expression, but I think this is actually more of a trap.
2. stick it in your mouth. Like, half an inch or so – so there’s a ‘pool’ of saliva that can form against your lips without running down inside the instrument. Sure, you’ll swallow between verses or something anyway – but anything you can do to keep it from getting excessively mucky down there will help significantly.
3. whack it. Having inspected the internals and found condensation beading on everything, I noticed I could dislodge the moisture by increasing gravity – ‘slinging’ it off to to say. What this amounts to is whacking it end-downwards on my knee when I start noticing tone interference, as a centrifuge is not readily available. I bring it down on my thigh above my knee sharply while pushing my leg up (in a seated position). The idea is that this slings the fluids down towards the bottom of the melodica – which I can then attempt to clear with the spit valve. I also do this before putting it away. When particularly annoyed I wack it towards a side too. It seems to address the worst of the problem and the notes sound more clearly.
Lastly, a proposal – what if we designed a mouthpiece with a spit collector – like a vacuum dust collection bin, effectively – only providing a place for excess salivations to collect rather than going down in the instrument? It would have its own valve on the bottom of it for lubricating the band room floor. 😉