My expectation is that you're suffering from situation described in JMeterSocketClosed wiki page.
If you're absolutely sure that you server is not overloaded and environment is healthy you can take the next steps:
Change implementation of all your HTTP Request Samplers to HTTPClient4. The best, easiest and fastest way to do it is using HTTP Request Defaults
In user.properties file (lives under /bin folder of your JMeter installation) add the next lines:
And in hc.parameters file (same location - JMeter's /bin folder) add the following line:
Hope this helps