Connection timeout Error

I would really appreciate it if someone can tell me why I'm getting the
following error when an embedded resource is retrieved with my sampler. The
error is in response to a request for .css and in another sampler a .js.
And it happens intermittently, not consistently. I have added a cache
manager and I'm clearing the cache for each iteration.
Is the connection really timing out or is it because of the firewall or
could it be some other reason?

Regards
Sam

org.apache.http.conn.ConnectTimeoutException: Connect to xxxxxxxxx timed
out at
org.apache.http.conn.ssl.SSLSocketFactory.connectSocket(SSLSocketFactory.java:551)
at
org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:180)
at
org.apache.http.impl.conn.ManagedClientConnectionImpl.open(ManagedClientConnectionImpl.java:294)
at
org.apache.http.impl.client.DefaultRequestDirector.tryConnect(DefaultRequestDirector.java:645)
at
org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:480)
at
org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:906)
at
org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:805)
at
org.apache.jmeter.protocol.http.sampler.HTTPHC4Impl.sample(HTTPHC4Impl.java:286)
at
org.apache.jmeter.protocol.http.sampler.HTTPSamplerProxy.sample(HTTPSamplerProxy.java:62)
at
org.apache.jmeter.protocol.http.sampler.HTTPSamplerBase$ASyncSample.call(HTTPSamplerBase.java:1804)
at
org.apache.jmeter.protocol.http.sampler.HTTPSamplerBase$ASyncSample.call(HTTPSamplerBase.java:1772)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source) at
java.util.concurrent.FutureTask.run(Unknown Source) at
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) at
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) at
org.apache.jmeter.protocol.http.sampler.HTTPSamplerBase$1$1.run(HTTPSamplerBase.java:1240)
at java.lang.Thread.run(Unknown Source)