Is your Viper balancing on the rear of the recommended CoG? If it is then that could certianly be the problem. Otherwise its simply too much rates and needs dropping down a hole or two on the servo arm.

On the note of balancing the Viper. Mine has come out profoundly tail heavy. With all gear in and 2lbs of old fashioned batteries in the nose I am still going to need to add lead, about 1lb I think. Anyone else had it come out this far out?