The Twin Frozr II models are generally awesome (Although newer models are a bit loud for my taste, the heatsink design is still solid). I LOVED my TFII GTX470 - It included an unified memory/circuitry heatsink, too. MSi has (From my experience) a painless warranty process. I had no problem getting my 470 repaired when a fan bearing went bad. It's silly to bash the manufacturer in this case - All AIB partners let a few defective cards out into the wild (I will not jump to a conclusion for or against PEBKAC at this time).
It is possible that your GPU, PCB, or some other circuitry was damaged during removal of the stock heatsink, removal/replacement of TIM, or installation of the AXP.
The type of TIM you're using may impact the success of a reapplication or heatsink swap - I would avoid using diamond based compounds (Like IC Diamond 7), particularly on AMD/ATi cards, as they may scratch the GPU IHS. From my experience, IC Diamond sometimes stains/scratches surfaces.
And of course throw all of your AS5 in the garbage, because conductive TIMs with cure times are so 1999