in Jmeter I have this situation:
What I want is that once the thread group is started, I want all threads to block (syncronize) after doing Loop1. Once the threads are all synchronized and blocked I would like to start a "sleep" phase for X seconds and then restart them all together
Take a look at Synchronizing Timer and Flow Control Action Sampler combination, example implementation:
More information: Using the JMeter Synchronizing Timer