This happened to me on my old 996tt. Ended up changing out the fuel filter and fuel pump. It was either one of the two, because it ran just fine after.

 

Car's fixed! Turns out it was just the cylinder 6 coil or plug that was bad. They're less than a year old, so likely one or the other was defective. Both looked brand new when they came out. Fortunately cylinder 6 is the easiest one to change, so it was really the best case scenario!