TempDB automatically re-uses space. So, if the transaction log continues to grow it is because it has rows which may not be deleted due to open transactions.
I have found that using connection pooling in ADO if you don't close your connection, the connection pool won't complete implicit transactions. This results in SQL Transactions that remain. Until the transaction is committed, or rolled back, the transaction log cannot re-use this space.
Since you say this only happens once a quarter I would guess the ADO issue may not be the cause. You may have a longer running job requiring more time and thus grows tempdb transaction log.
If you find this is the case, then consider taking work for larger jobs and breaking them down into smaller batches.
Finally, make sure CheckPoint is at a short enough interval. This is what causes SQL Server to go and remove completed transactions from transaction logs of databases in simple recovery mode. TempDB is always in simple recovery mode.