I would defrag the tablespace/datafile.

use these two statements. the first will tell you what tablespaces are fragmented:

select tablespace_name,
total_extents,
percent_extents_coalesced
from dba_free_space_coalesced
where percent_extents_coalesced<> 100;

alter tablespace (tablespace_name) coalesce;

see if that helps at all.
you should probably do an export/import as well.

- Magnus