Sadly, memory allocation has bugs at present. Boilerplate:
Please note that, at present, the String library has bugs as discussed here and here.
In particular, the dynamic memory allocation used by the String class may fail and cause random crashes.
I recommend reworking your code to manage without String. Use C-style strings instead (strcpy, strcat, strcmp, etc.).
Even if you code perfectly, the memory bugs will bite you.
I would be happy to make constructive suggestions if I had a better idea of what you are trying to do with your code.