I had a Nikkor 28mm f/2.8 aperature gum up three times like that. I took it to two different shops for CLAs. The first shop only fixed it for 6 months, then the same problem. Shop #2's fix only lasted for about three months. I took it back, and he re-did the job, but again, much less than a year of use. The third time I just bought a replacement lens, as it was cheaper than infinate repair visits. BTW, it was the only lens I ever bought from abroad, so no US warranty.