PTC plugin causing memory leak.
It's a very strange pattern:
First we see "API Exception caught" like the following

INFO: [xUnit] - [MSTest-Version N/A (default)] - 1 test report file(s) were found with the pattern 'TestResults*.trx' relative to 'D:\Jenkins_ROOT\NGMP.DIRECT.Sandbox\SWProduction\TestLibraries\TestlibrariesCommon\CommonSwitchingPaths' for the testing framework 'MSTest-Version N/A (default)'.
Jun 04, 2014 4:54:30 AM [INFO] IntegritySCM - Database shutdown successful!
Jun 04, 2014 4:54:31 AM [INFO] IntegritySCM - API Exception caught...
Jun 04, 2014 4:54:31 AM [INFO] IntegritySCM - Failed to establish a session: Session not authenticated/authorized.
Jun 04, 2014 4:54:31 AM [INFO] IntegritySCM - API Exception caught...
Jun 04, 2014 4:54:31 AM [INFO] IntegritySCM - Failed to establish a session: Session not authenticated/authorized.
Jun 04, 2014 4:54:31 AM [INFO] IntegritySCM - API Exception caught...
Jun 04, 2014 4:54:31 AM [INFO] IntegritySCM - Failed to establish a session: Session not authenticated/authorized.
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - java.util.concurrent.ExecutionException: java.lang.Exception: Failed to create APISession!
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at java.util.concurrent.FutureTask$Sync.innerGet(null:-1)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at java.util.concurrent.FutureTask.get(null:-1)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.scm.IntegrityCheckoutTask.invoke(IntegrityCheckoutTask.java:359)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.scm.IntegrityCheckoutTask.invoke(IntegrityCheckoutTask.java:25)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.FilePath.act(FilePath.java:920)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.FilePath.act(FilePath.java:893)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.scm.IntegritySCM.checkout(IntegritySCM.java:789)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.AbstractProject.checkout(AbstractProject.java:1251)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:604)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:513)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.Run.execute(Run.java:1706)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.ResourceController.execute(ResourceController.java:88)
Jun 04, 2014 4:54:33 AM [INFO] IntegritySCM - at hudson.model.Executor.run(Executor.java:231)

than

Exception in thread "Executor #1 for master : executing NGMP.Direct-Library.Fixture4140Flat #1352" java.lang.OutOfMemoryError: unable to create new native thread
at java.lang.Thread.start0(Native Method)
at java.lang.Thread.start(Unknown Source)
at hudson.remoting.AtmostOneThreadExecutor.execute(AtmostOneThreadExecutor.java:88)
at java.util.concurrent.AbstractExecutorService.submit(Unknown Source)
at jenkins.util.AtmostOneTaskExecutor.maybeRun(AtmostOneTaskExecutor.java:84)
at jenkins.util.AtmostOneTaskExecutor.submit(AtmostOneTaskExecutor.java:72)
at hudson.model.Queue.scheduleMaintenance(Queue.java:928)
at hudson.model.Executor.run(Executor.java:265)