I also noticed a problem around the time of the introduction of gun powder, I think it occurs with one of the other races, because it happens after you click to end your turn, I thought Cuba was the problem because both games this happened in, I had Cuba as an opponent where I usually don't. So I removed Cuba from my game, and have not had the problem since, but I don't know for sure if that was the problem because it could have been another race causing the problem. I will let you know if it happens again without Cuba in the game.
I also have noticed a problem with the Northrup jet, I think it is a prototype because you only get one, just as you only get one red baron plane. Once I get that jet, if I try to use it, like move it to another city or anything, the game crashes. This has happened to me several times, so what I do now is, when I see I have that one jet, I immediately click on the skull and crossbones and delete it, and the game runs fine then. Anyone else notice this problem?