From: "Carl Cadwell" <CarlC@Cadwell.com>

Perhaps you experience is similar to the following:
A discussion on the list in the past described a similar event that was
characterized as an engine hesitation.  I experienced it frequently.
It happened primarily when switching tanks the first time, usually on
hot days and above FL140 and especially at FL 230-250.  The fuel
pressure dropped, the engine lost power, and then returned to normal
much more quickly than the pilot.  

The problem was attributed to warmer fuel in the line in the cabin being
vaporized as the electrical fuel pump sucked it up.  The slight
additional negative pressure was enough to cause the warmer fuel to
vaporize.  The problem was solved by moving the fuel pump from the
sidewall to the floor so the pump always pumped.

Carl Cadwell
IVP, N25CL, 300 hours, 150 hours with no more hesitations.