FAULTY SONY

I got my Sony Z3 comp on 11Nov and since then i had to sent it in 3times, currently in for the 4th time. The phone keeps on restarting and wont unlock at times. What is more stupid is that after receiving it back the 1st time the cover of the WATERPROOF Sony wasn't replaced correctly. How can a phone be waterproof if the cover got a gap? Last time 3rd party said No Fault Found, day later phone restarts again. NEVER BUYING A SONY PRODUCT AGAIN!Since I got the sony I could hardly use it, has been in for repair long periods of time.