Tammy, the chocolate mint does sort of the same thing. I'm new at it of course... dug some out of various generous folks' beds, potted it up in containers, and have brought them in.
The die-back comes and goes in phases it seems. Sometimes it will look like it's all dead, but then here come sprigs up from the dirt, or sometimes from the dead-looking stems - just like Deborah L says. I haven't been able to identify the trigger(s). I have a tendency to overwater (just ask my gazania), so that's a possibility.
I'm going to plant
one of them outside in a "safe" place (= buried bucket) tomorrow, just to see what happens to that one.