It's been years since I discovered that old "postage-stamp" micas go bad. I have seen cheap foreign-made ceramics spit molten metal
out the side as well, thanks to a cheapskate parts buyer where I used to work. Rule of thumb here (and my best freind disagrees): replace ANY mica over 50 years old, and be ready to replace some ceramics, too, Just try and match the temperature coefficient when doing RF/IF circuits (NPO = C0G these days). It doesn't hurt to go one step higher on voltage rating, either (if size allows). There's a great seller on eBay I HAVE NO CONNECTION TO called Tracktronics that always seems to have a large selection of HV ceramics on hand. Mostly RMC brand, made in America, can't beat that!