What I'm trying to do is to run multiple threads at some starting time. Those threads must stay alive for 90minutes after start. During the 90minutes they execute something after a random sleep time (ex: 5minutes to 15minutes). Here is a pseudo code on how I would implement it. The problem is that doing it in this way the threads run in an unexpected way.

How can I implement correctly something like this?

Class MyJob {
  static triggers = {
     cron name: 'first', cronExpression: "0 30 21 * * FRI"
     cron name: 'second', cronExpression: "0 30 19 * * FRI"
     cron name: 'third', cronExpression: "0 30 17 * * FRI"

  def myService

  def execute() {
    switch( between trigger name )
      case 'first':
        model = Model.findByAttribute(...)
        ...
        myService.run( model, start_time )
        break;
      ...
  }
}

class MyService {     

  def run( model, start_time ) {
    def end_time =  end_time.plusMinutes(90)
    model.fields.each( field -> Thread.start { executeSomeTasks( field, start_time, end_time ) } )
  }

  def executeSomeTasks( field, start_time, end_time ) {
    while( start_time < end_time ) {
       ...do something ...
       sleep( Random.nextInt( 1000 ) );
    }
  }
}