my money is on the ESC. my Viper VTX8 just did the exact same thing. I went with everyone's suggestion and got a new motor. including the guy on here from viper said he was sure it's not a esc issue.
well it was the ESC. $160 down the tubes on a new motor.
http://www.rctech.net/forum/13744648-post887.html
you're like the 3rd person in a few weeks that's had this happen.