i had a similar problem (minus the hang-ups). Sprint replaced my phone after witnessing the issue. THEY WOULD NOT DUPLICATE THE ISSUE ON MY FIRST VISIT THOUGH, and insisted that nothing was wrong with my phone. make sure the phone is exhibiting the problem right before you go in, and tell them that the problem you just had "looked at" is repeating and you need something done about it. Insist on showing the issue to the tech (or else they will just hook your phone up and tell you there's nothing wrong with it again).
Apparently, the tech's are REQUIRED to use their "slate" software at Sprint. A certain percentage of the phones must have been tested in order for them to "be doing their jobs properly." The problem with "slate" is that 95% of all phones will "pass post" even if there is a serious short within the phone (which is what it sounds like you are experiencing).
Be persistant, but be nice. These techs have people yelling at them all day long. . .make them want to help you and I promise it will work. My tech told me pretty simply, "when people show me the error, even if it doesn't show up in testing I can usually just replace the phone. . .if they are nice. . .i'll replace it on the spot. If they are d**ks, I'll make them wait 3-5 business days."
hope this helps