Non HTTP response message: The target server failed to respond: Is my server failing to handle load

It looks like that you're facing the problem described in Connection Reset since JMeter 2.10 ? Wiki article.

If you upgraded recently to JMeter 2.10 or 2.11, you may have noticed increased errors of this type:

Socket closed

Non HTTP response code: org.apache.http.NoHttpResponseException (the target server failed to respond)

So

1. Change "Implementation" of your HTTP Requests to HTTPClient4

2. Add the next 2 lines to user.properties file (usually lives under /bin folder of your JMeter installation)

   httpclient4.retrycount=1
   hc.parameters.file=hc.parameters
   

3. In hc.parameters file (also lives under JMeter's /bin folder) uncomment the following line:

   http.connection.stalecheck$Boolean=true
   

For more information on different JMeter properties and ways of setting and altering them refer to Apache JMeter Properties Customization Guide