A serial update might work for this, but the implementation would require me to see the underlying DDL for the table and indexing, specifically the clustered index.
For the proper rules to doing something like this, do a search for 'Quirky Update' here on SSC and you'll find a very detailed article on how to do something like that.
EDIT: found it: http://www.sqlservercentral.com/articles/T-SQL/68467/
Other than that, you'll have to make data-islands of the data and then perhaps setup a row_number() over on the subset. I'll have to chew on that one a while.
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