At -15 that seems normal. What I do with mine when it gets that cold is just hold the starter on, if it starts it'll kick the starter out anyway and not hurt it. Holding the key an extra couple of seconds 99% of the time will keep my nytro running on the first try.
If I let off the key when it sounds like it's running it'll usually stall when it's -15 outside. From what I have read that seems normal. The holding key trick seems common on here as well, works for me.