RE: Viper 500 flight problem
Mine did the same thing: CG at 3" and a Jett Q500 engine made for a very interesting first flight... As long as it was tracking straight, it would keep on doing so, but as soon as I touched the stick it would start to jump all over the place. It really looked like it got into some sort of oscillation.
After the Jett engine "removed" the firewall, I moved the engine 1/8" forward, moved the battery into the tank area and glassed the tail surfaces.