Query optimizer in 5.01 Informix

Has anyone noticed the 5.X optimizer making bad choices when choosing a query plan? In my case the optimizer insists in performing a sequential read of a detail table ignoring the unique index used to join to the master. Performance, as you can imagine is very, very poor. Informix has not given us information as to whether this is a known bug, but they seemed supprised. I've seen it in several different forms over the last month or so, and I wonder how many other people have the same experience... JB