Faulty BEGIN COMMIT on Informix 4.1

Need assistance from Informix gurus out there! Is there any guarrantee that statements inside a BEGIN-COMMIT would be done atomically. We are using Informix 4.1 on top of AIX 3.2, and we have discovered that several times the BEGIN-COMMIT in our programs missed a statement or two without warning to users or rolling back the transactions. It happens randomly both on small and large BEGIN-COMMIT block. True this is rare, but we want to make sure that it never happens at all. Your help would be appreciated. Thanks, andro@ibm.net