I had the same problem with a lens.  I took mine apart and fixed it.
It was a nerve racking four hours before I could finally reach an
acceptable level of clean.  Without a dust free environment, it is
virtually impossible to properly clean inside a lens.

If you decide to do this job yourself, then be prepared to have a lot
of patience.  Lay the parts out on a white cloth or paper, so that they
are clearly visible and don't get lost.  Try to photograph each step of
the process to make sure that you don't put a lens element in
backwards.  I purchased a rubber bulb air blower from the pharmacy to
blow the dust away.  I also used the lenspen cleaning system for any
fingerprints, etc.  If you want to use your bare hands to do this job,
then wash them in dish detergent, to remove as much oil from your hands
as possible.  It is a very frustrating job and you usually get more
dust in the lens than you take out.  Be patient, take your time,
document each step and you should be able to do this job yourself.  I
used a halogen desk lamp to illuminate the lens as much as possible, so
that I could see the dust floating around in the air.  Those home air
purifiers help a little bit but there will always be dust floating
around in the air.

Professional photographers would just send the lens back to a proper
repair depot and pay the bill.  My personal approach is that I love to
take things apart and hopefully get them back together in one piece.
Every person is different and each person much know his abilities.  If
you are up to the challenge and short on cash, then make a decision and
go with it.  I've also rebuilt many of my older digital cameras when
they failed.  That is my nature.  I want to find out what failed and
why?

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