open session failure

Hi I am issuing some commands to my device via NI-VISA through TCP/IP.

Here is what i do:

1.open session

2.some application specific commands

3.close session

These three steps are done multiple times and it seems most of the time the communication is fine. Once in blue moon i get an error from ni visa : 

Failed to open session - Check TcpipSocket connection at address x.y.z.a(Failed command: OpenSession)

I check the device and it is running on the ip  x.y.z.a, i close ni visa and reconnect to the same ip and issue *idn? i get a valid response back. I dont understand what is causing the open session to fail intermittently

thanks