I tried everything to fix my flashed 360 when it went RROD. Nothing worked so I took a chance and registered for the warranty on xbox.com. About a week later I got the coffin, and about two weeks after that, a replacement 360 arrived.
I had broken the Microsoft seal, taken the thing completely apart, wanged around all over the inside, tampered with the firmware, and they still replaced it. I don't believe they even check the things anymore. Just ship a repaired one out and send the broken one to Mexico for repair. Take a chance, the worst that can happen is you'll have to pay return shipping.
Good luck!