,

(SUM(unallocated_extent_page_count)*1.0/128) AS [free space in MB]

FROM sys.dm_db_file_space_usage;

SELECT DB_NAME(dbid) AS DBNAME,

(SELECT text FROM sys.dm_exec_sql_text(sql_handle)) AS SQLSTATEMENT, *

FROM master..sysprocesses

WHERE open_tran > 0

here are some things that might be nice to track during the time the tempdb is growing, just make a job and some tables and run these into tables every min and it should give an idea to what is causing your pain...