I'm no expert, and really hope that I'm wrong, but I've read a lot about this issue over the years, and it sounds like a textbook description of an IMS failure.
![Frown](http://www.986forum.com/forums/images/smilies/frown.gif)
They can go without warning, without noise, and over half the people I've heard about that had the failure had it when they were stopped or coming to a stop.
178,000 is pretty mature for any car, but especially one of these.
Best of luck on this. I hope it turns out to be something trivial.