I've tried - the finest small soft artist's brush, distilled water, the slightest touch with wet bristles near parallel to the diaphragm -? the answer, for most of us mortals, is... no. Gold comes off. And from your photos, the capsule doesn't look that bad for its age, nor do I think it's the capsule that would cause the problem you're observing.
I would use a blower brush (for camera lenses) and VERY GENTLY remove any loose stuff.? I would NOT USE ANY LIQUID or ANY HIGH PRESSURE AIR.
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Yes, you might remove some Gold but actually the capsule will work fine with a bit of Gold missing :)? The real danger is your liquid moving yuck behind the diaphragm.? Then the mike will be forever / intermittently noisy. :(
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I think Josephson or one of the other commercial mike makers said you DON'T TRY CLEANING CAPSULES UNLESS YOU CAN REPLACE THE CAPSULE IF IT ALL GOES TO SHIT.? I second this as an ex commercial mike maker.
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From your description, I don't think you have a dirty capsule.? This would be VERY NOISY long before it stops working.
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I think the muck on your PCB coupled with humidity, was making the polarizing voltage generator wonky.? After 3 min. the warmth of the electronics dried the stuff enough for it to work again.? The polarizing voltage has some serious decoupling to remove any noise in the generator.
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In da 80s, I didn't like ANY of the conformal coatings for the HiZ part of any PCB.? Modern stuff might be OK but I have no experience.
