Use groovy to run sequentialy loadtests

I use soapUI 5.2.1.

I have TestSuite with 4 TestCases and each TestCase has N test steps. 

For each TestCase I define one loadTest.

My goal it's to run loadtest on each test step to get some stats about the perf of the request.

To do that, I use groovy script (run 4 TestCases) :

import com.eviware.soapui.impl.wsdl.loadtest.strategy.*
import com.eviware.soapui.support.xml.*
import com.eviware.soapui.config.*
import com.eviware.soapui.model.settings.Settings;
import com.eviware.soapui.settings.HttpSettings;
import com.eviware.soapui.impl.wsdl.loadtest.*;
import java.util.Date;

//define constants
final int NB_THREADS = 20;
final int NB_TIME = 300;
def startDate = new Date();
final String PATH_RESULT = "result_"+startDate.format("yyyy-MM-dd-HH-mm")+".txt";

// create groovyUtils and XmlHolder for response of Request 1 request
def groovyUtils = new com.eviware.soapui.support.GroovyUtils( context );
def testCases = context.testCase.testSuite.getTestCaseList();


//create burstStrategy
def xmlobjBuilder = new XmlObjectConfigurationBuilder();
def xmlobj = xmlobjBuilder.add("burstDuration", "10").add("burstDelay", "0").finish();

fos = new FileOutputStream( PATH_RESULT, false )
String conf = "THREADS : " + NB_THREADS + " TIME : " +  NB_TIME + "\n";
fos.write(conf.getBytes());
for (def testCase : testCases){
	if (testCase.getLabel().equals("PERF-SIGDOC-RUNNER")){
		break;
	}
	def loadtest = testCase.getLoadTestByName("perf");
	log.info(testCase.getLabel());
	fos.write(testCase.getLabel().getBytes());
	fos.write("\n".getBytes());
	def testSteps = testCase.getTestStepList();
	//disable all teststeps
	for (def teststep : testSteps){
		teststep.setDisabled(true);
	}

	//call loadtest
	int i=0;
	for (def teststep : testSteps){
		teststep.setDisabled(false);
		//set loadtest strategy
		loadtest.setHistoryLimit(0);
		loadtest.setLimitType(LoadTestLimitTypesConfig.TIME);
		loadtest.setTestLimit(NB_TIME);
		loadtest.setThreadCount(NB_THREADS);
		Settings settings = loadtest.getSettings();
		settings.setBoolean( HttpSettings.INCLUDE_REQUEST_IN_TIME_TAKEN, false );
		settings.setBoolean( HttpSettings.INCLUDE_RESPONSE_IN_TIME_TAKEN,false );

		
		def burstloadStrat = new BurstLoadStrategy(xmlobj, loadtest);
		loadtest.setLoadStrategy(burstloadStrat);
		
		//run loadtest
		WsdlLoadTestRunner loadTestRunner = new WsdlLoadTestRunner(loadtest);
		loadTestRunner.start(true);
		loadTestRunner.waitUntilFinished();
		
		//write TPS
	     def statisticsModel = loadtest.getStatisticsModel();
	     def data = "\t"+statisticsModel.getValueAt(i, 1) +";" +statisticsModel.getValueAt(i, 7) +";" +statisticsModel.getValueAt(i, 10) + "\n" 
		log.info(data);
		fos.write(data.getBytes());
		teststep.setDisabled(true);
		i++;
		loadTestRunner.release();
	}
	loadtest.release();
}

fos.flush();
fos.close();

With this script , I have a problem . After a moment , the performance fall to zero in terms of TPS stats.

After using the script, I raised the loadloat (without groovy) on TestSTEPS that have inconsistent values ​​, and I get consistent values ​​.

Does anyone has any idea about that ?