error -691 with ISAM error -143

Hi: I've been getting informix(-691) isam(143) errors when three or more clients are running large transactions at the same time. Each transaction is about 100 inserts followed by an additional 100 inserts that are constrainted by the first 100 inserts (a one to one relationship). tbstat -uk shows a process is locking itself out. Informix says the contention is caused by contention of a hashing values' locks and that this is an undocumented feature. Has any body heard of this feature???