Hi Rajiv Gujral,
Please check message flows for your vendor. You could find witch timer is expired. I saw sometimes, that for Siemens equipment, if expires timer T7, there is problem with wrong definition of CGI in source/target BSC/MSC. It is possible to see in counters.
That my recommendation: At firs check if there are right definitions of handovers. Not only in BSC (target cells), but also in MSC (outer cells) in both directions.