Tach problem

2001 aurora 3.5. I love this car but it has developed a problem. Every once in a while it will shudder and act like something has slipped. The tach wil peg out but the car continues to drive well and it is not over reved. When you shut the car off and restart everything is normal. Where should I start?
Futher develpments. Code P0385 set. Crankshaft position sensor. How hard is this to change?

Changed the cranckshaft position sensor and problem has gone away.

I have the same problem. Except that mine happens normally when the car is warming up. The ECT is normally between 150-195, when this happens.
Did you hear anything about this. I like my car a lot but I have a lot of little annoying issues like this.