TSIO 550 guys and or smart engine guys:

I am having a vibration issue and want to see if anyone has ideas on what to look for.  Basically, at takeoff and climbout to around 10,000’, the motor and prop are very smooth and strong.  There is no vibration (prop very recently balanced) and all numbers are normal.  As I pass above 10k to the low flight levels, a vibration in the motor starts to develop, and when I level off and set the engine for cruise (LOP, 18gph, 34 MAP, 2500 RPM) the vibration is at its worst.  It’s almost like the prop is out of balance, which is why I had the prop balanced at 2500 rpm.  Speeds are good and all engine numbers are normal, no changes.  This will remain for an hour or 2, then the motor seems to magically smooth itself out. On descent and landing, motor seems smooth and all numbers are normal. Very recently, on a 4.5 hour flight, about 3.5 to 4 hours in the vibration slowly came back.  I don’t know what to blame, and my motor mechanic is looking at me funny.  I should note that at no time do I think the vibration is horrible or unsafe, just annoying.  I can feel it in the stick and see it in the panel when its happening.