I had the same problem with my monti caps when I was in Texas. They would grow out and then, mysteriously, die off. They would get a brown looking slime on them and start receding. Then they would start growing back. I did a bunch of research and swapped pictures and ideas with some people in Dallas for a couple years and never really found the root cause. The two most logical ideas were stray voltage and toxic warfare between softies. That's back when I had the large leathers so there was a good chance that toxic warfare was the culprit. Like you, the only thing affected was the monti caps. All my other SPS pieces did great. Good luck! Keep us posted if you figure it out.