In Android, threads and timers are not reusable, so the thread in your timertask can't be reused. Every time you want to re-launch your TimerTask, you need to instantiate a new one.
So add these lines
Tempogatto = new Timer();
timerTempogatto = new timerTempogatto();
before this one
Tempogatto.schedule(timerTempogatto, 0, 20000);