Hi

Look at your alert log and see at approximately what time this is happening.Try to find out if there are any dbms_jobs that are running at thsi time.

schedule the following sql's to run and see that you spool it to a file or something at the time mentioned in the alert log

Code:
select	s.sid || ',' || s.serial# sid,
	s.username,
	u.tablespace,
	substr(a.sql_text, 1, (instr(a.sql_text, ' ')-1)) sql_text,
	u.blocks/128 mb,
	((u.blocks/128)/(sum(f.blocks)/128))*100 pct
from	v$sort_usage	u,
	v$session	s,
	v$sqlarea	a,
	dba_data_files	f
where	s.saddr = u.session_addr
and	a.address (+) = s.sql_address
and	a.hash_value (+) = s.sql_hash_value
and	f.tablespace_name = u.tablespace
group by
	s.sid || ',' || s.serial#,
	s.username,
	substr(a.sql_text, 1, (instr(a.sql_text, ' ')-1)),
	u.tablespace,
	u.blocks/128
Please check if the script works as desired in oracle8.I dont have acesses to a oracle 8 database right now.

regards
Hrishy