I've accepted answers in 18 of the 20 questions I've posted to SO; but my rate is stuck at 82%, not 90%.
Is this an infrequently run batch process or has something caused it to stop running?
It's updated periodically. No need to worry.
Update: Actually, I just looked and it seems you have 21 questions. 2 don't have any answers so they don't count. 2 aren't old enough so they don't count either. That leaves us with 17 eligible questions. Of those all but three have accepted answers. That works out to 14/17 or 82%, so I guess you are stuck until you you accept an answer on one of these:
Please see: Question about Accept Rate
And on the blog, it it mentioned by Jeff that the accept rate is a heavily cached feature, so it will take some time to change if you have made recent changes.