My wife's MP5 automatic is having the same problem.
It doesn't happen every day, just sometimes, and it's never happened to me.
She gets in, turns the key, hears the relay click from the glove box area, and that's it.
Sometimes she will put it into neutral and then it starts, but today, putting it into neutral didn't help.
When it finally does turn, it has plenty of power, so I don't think it is the battery.
I also have a battery load tester and it always passes, so I really doubt it's the battery.
BlackWagon, if you find out what is causing this, please let me know.
I'd really hate for her to get stuck in the winter time.
Thanks