There's definitely something suspicious about that. The largest value you can store in a java Double is somewhere around 10^307, if I have that right. Not too far from that anyway. But you're getting a number around 10^312625 in scientific notation there. That's a long way out of the Double range, and it's an understatement to say that.
It's also kind of suspicious that the string "12625" occurs both before and after the "E".
And you said the error occurs sporadically -- that could mean you have threading issues. Java formatting objects are notorious for not being thread-safe, so perhaps you're using more than one
thread to do this XML serialization?