Assuming everything is correct,wing warp, controls,motor mount etc. The only thing I can think of is that even if the plane lifted off, it still could have snap rolled on you.Have you tried a hand launch to see what it does? If two much up elevator is used on take-off it can get airborne before it has sufficent flying speed to substain flight, followed by a snap roll.Just one more thought.Try a hand launch next time with a gentle climb out and see what it does.
Finally, you may still need a little more nose weight , balance level may still be tail heavy for this plane, some of the manufacturers are a little off on there recommended balance point.If you read any of the GWS P-51 threads, we had a learning curve on that airplane to find the best balance point.It really sounds to me that the airplane lifted off to early and snap rolled in.Classic symtoms.
Good Luck, and let us know what you find.