temp tablespace full

I've got a strange problem. Even after a restart of Oracle (Oracle 10g,
10.1.0.2.0, AIX 5.x) the temp tablespace (8 Gigs in size) remains full
and doesn't seem to be purged. At least this is what the Enterprise
Manager tells me.
When I try to create a table using 'create table xxx as select ...' the
creation always fails because the temp tablespace cannot be extended.
I'm absolutely certain that my create statement won't consume 8 Gigs of
temp space.
Has anybody a clue on how to solve the problem?
Thanks in advance.

sim

.