AI sometimes slow to come up to speed

My attitude indicator was last replaced back in November 2006 with a
Mid-Continent overhauled unit. Since then I have flown on average
twice every single week of flights an hour or less in duration. Thus,
there are about 180 hours or so on this air-driven AI but it is out of
warranty.

Around ten flights ago I noticed the AI, which normally rests in
almost level mode and spins right up at avionics-on, was resting with
an indication of 50 degrees off level. When I started the aircraft
and turned on the avionics the AI remained this way for perhaps 5
minutes before finally migrating to a fully erect and level mode.
The unit remained proper for the rest of the flight.

For the next three flights the AI spun right up when the avionics were
switched on but this behavior again returned twice over the following
five flights. So far it has not happened on two consecutive flights.
The aircraft is equipped with a back-up electric AI.

Is this a sign that the AI needs to be replaced again so soon after it
was installed or could this be something related to the air filter,
pump, or piping connecting the AI to the pump? The fact that the
problem is not happening every flight makes me wonder if this issue is
not directly caused by the AI dying. The only thing keeping me from
replacing it without question is its relative young age and low usage.