16 KB page size is ok and is maximal for 

FB1.5 to FB3.

During problem time run gfix -h and show us the results and also gstat results for that tabe and indexes on it.

P.S. to recreate index you do not need to set it inactive. You can activate already active index. But this operation can be done (safe) only when noone else work on that database.

Also running -mend without looking about errors on the database is not good option, as you can loose same data.

Regards,