For the last several weeks and on the first-start-of-the-day engine starts only, I've been seeing a smooth running engine for 2-3 minutes, then it suddenly starts running rough.  It does this for about 30 seconds or so and then goes away.  During the event, the MP starts to climb (I've seen it go as high as 20") while the RPM decreases or stays fairly constant at 1000 or so.  Mixture rich or lean for taxi seems to have no affect.  Fuel boost pump on doesn't change anything either.  After a minut