3 back boxes since september ohh I love my car lol

[twistedl]

I am having a very annoying problem, I brought the car in oct and found out it had a new back box fitted in sept. I kept gettting a gay noise and found out my middle box had gone. Fair enough I said wacked a new 1 on. All was ok and then I noticed another blowing noise and found out the back box had broken at the weld to the middle box keep in mind this exhaust was only a month and a bit old so I got my money back and went elsewhere. After 2 weeks the new back box does the same thing so aggain I got that replaced as the pipe was knackerd in the exact same place. I have now had the new b box on for about a week and I have noticed a slight blow and backfire. The blow is at the same place in the rev range ( near 3k rpm) so I thought i would check the exhaust by covering it with my hand so its sealed and it makes a hiss noise indicating a blow. Also when I rev it in neutral above 3.5k as it comes down it make a very tiny backfire.

Any ideas would be apreciated as I cant seriously have had 3 faulty back boxses. People have sugested engine mounts but the car doesnt rattle at all if that helps.