Why are you changing to SIMPLE mode? Just because the transaction log is growing isn't really a good enough reason. Once you change to SIMPLE mode, you no longer have the ability to restore to a point-in-time.
If you are doing reindexing, the transaction log will grow large regardless of what mode you are in. Why? Consider the reindex as a transaction, until the reindex is done (committed), it needs to be able to rollback the transaction.
Are you set for FULL Recovery mode and doing frequent transaction log backups or have you just been doing full backups?
-SQLBill
Posting advice: FAQ481-4875