You should get rid off anxiety including performance anxiety and stress as those are main offenders in many.You can do squeeze and stop technique.You can stimulate your partner to maximum before getting yourself stimulated.You can deviate your thought to something nonsexual when you about to near ejaculation.You can put on a condom or condom with anesthetic which will decrease the excessive stimulation on penis and there by prevent premature ejaculation.If all these not helping then you can approach a psychiatrist for behavioral therapy and medicinal management like antidepressants.SSRIs like Paroxetine or dapoxetine are usually prescribed for this.Balls sagging and tightening is due to temperature difference of body.Testis needs lower temperature than that of body.So when body temperature increases it sags ans when drops it tightens to body.That is normal process.Hope this may help you. Let me know if you have any further query.