webtests fail erratically and with no apparent reason

I am using webtest plugin for an AppFuse 2.0 project. We have over 100 tests.

When running tests in my windows box, all runs fine.
Sometimes, when running the same tests within Jenkins, they seem to fail without any apparent reason and randomly:

ERROR (com.gargoylesoftware.htmlunit.javascript.background.JavaScriptJobManagerImpl) - Job run failed with unexpected RuntimeException: Exception invoking jsxFunction_getElementsByClassName
java.lang.RuntimeException: Exception invoking jsxFunction_getElementsByClassName
at net.sourceforge.htmlunit.corejs.javascript.MemberBox.invoke(MemberBox.java:198)
at net.sourceforge.htmlunit.corejs.javascript.FunctionObject.call(FunctionObject.java:479)
at net.sourceforge.htmlunit.corejs.javascript.Interpreter.interpretLoop(Interpreter.java:1702)
at net.sourceforge.htmlunit.corejs.javascript.Interpreter.interpret(Interpreter.java:845)
at net.sourceforge.htmlunit.corejs.javascript.InterpretedFunction.call(InterpretedFunction.java:164)
at net.sourceforge.htmlunit.corejs.javascript.ContextFactory.doTopCall(ContextFactory.java:429)
at com.gargoylesoftware.htmlunit.javascript.HtmlUnitContextFactory.doTopCall(HtmlUnitContextFactory.java:269)
at net.sourceforge.htmlunit.corejs.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3162)
at net.sourceforge.htmlunit.corejs.javascript.InterpretedFunction.call(InterpretedFunction.java:162)
at com.gargoylesoftware.htmlunit.javascript.JavaScriptEngine.callFunction(JavaScriptEngine.java:559)
at com.gargoylesoftware.htmlunit.javascript.host.xml.XMLHttpRequest.setState(XMLHttpRequest.java:181)
at com.gargoylesoftware.htmlunit.javascript.host.xml.XMLHttpRequest.doSend(XMLHttpRequest.java:525)
at com.gargoylesoftware.htmlunit.javascript.host.xml.XMLHttpRequest.access$000(XMLHttpRequest.java:64)
at com.gargoylesoftware.htmlunit.javascript.host.xml.XMLHttpRequest$1.run(XMLHttpRequest.java:461)
at net.sourceforge.htmlunit.corejs.javascript.Context.call(Context.java:537)
at net.sourceforge.htmlunit.corejs.javascript.ContextFactory.call(ContextFactory.java:538)
at com.gargoylesoftware.htmlunit.javascript.host.xml.XMLHttpRequest$2.run(XMLHttpRequest.java:467)
at com.gargoylesoftware.htmlunit.javascript.background.JavaScriptJobManagerImpl.runJob(JavaScriptJobManagerImpl.java:226)
at com.gargoylesoftware.htmlunit.javascript.background.JavaScriptJobManagerImpl.runSingleJob(JavaScriptJobManagerImpl.java:307)
at com.gargoylesoftware.htmlunit.javascript.background.JavaScriptExecutor.run(JavaScriptExecutor.java:182)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.NullPointerException
at com.gargoylesoftware.htmlunit.javascript.host.html.HTMLDocument.jsxFunction_getElementsByClassName(HTMLDocument.java:1018)
at sun.reflect.GeneratedMethodAccessor109.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at net.sourceforge.htmlunit.corejs.javascript.MemberBox.invoke(MemberBox.java:172)
... 20 more

If I patch webtest-maven-plugin's pom replacing htmlunit version 2.8 with 2.9, everything runs fine.