I think its like Madsen said its in the pouring.cut the flow down and see if that works and raise the pot heat,dont worry about frost.have you tried holding against spout and then lifting to get a puddle.it maybe splashing inside.I have been working on that.I just work on it till every thing works.
some people get frustrated and angry,which makes it worse.I have been casting since 1937,first with gilbert toy soldiers.I have singles from Ideal and modern bond double,lyman 4s and lee 1,2 and 6.and a bunch of others going into 1860s.

I bottom pour cast with a Lee custom mold slightly larger than the mold in question and think the mold and alloy is not hot enough. The foward part of the bullet is cooling as the aft is still molten. Because of the volume of the cavity, condensation is rapidly forming which creates the "bubble" cavities that cause the bullets to look corroded. A slow flow from the pot can cause splashing which creates small air cavities also. If you operate the mold at a temperature that is just shy of and to the point of frosty bullets the problem will resolve itself. The mold blocks must be hot to keep the entire cavity molting until the source of heat, the pour, is removed.

Of course, you get to a point were bullet stick or won't drop from the mold because the blocks are too hot. Simply cool your mold by placing it on a damp rag that you keep near the casting site. I develop a rhythm of X amount of casting cycles and Y amount of seconds with the mold on the rag. It makes casting very consistent because you are actually controlling your molds temperature across a very narrow range.

I use Lee molds exclusively and cast from about 30 special order molds for a wide range of calibers.

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Michael