Thanx for your comments.
Im using gdb. I'll update the results of dbx.

I can not use narrow down method u have suggested, as i can not
reproduce problem on my machine. Im facing problem on client side.

One more observation is :
Process is showing very abrupt behaviour.
Last time I faced similar problem on windows 2003.
On 2003, process was showing abrupt behaviour only in case of heavy
memory requirement. But after building application using .Net compiler
(VC7) instead of VC6, problem *disappeared*.
Now on AIX 5.1, memory requirement of process is huge. Do i need to
build application with other compiler? Im usin xlc compiler.
Because same application is running fine on windows .