VUser terminated

Hello All,

I am executing UFT scripts on a Virtual client.

Environment:
UFT 11.53
Windows 7
IE 8

I use the RuntestSet.exe , HP's utility program to execute the test sets.
I execute nearly 350 scripts per day.

sometimes, the execution halts in the middle . When I explored temp directory to see what the error is I saw the following

output.txt:
Vuser Terminated.

it will be of great help if you can give me an insight to this issue.