I am thoroughly familiar with how the helicopter works and checked everything three times.
I don't think that statement applies here. You missed something. I've set up at least 30 of these machines and can tell you that something isn't right. Is the tail rotor rotation in the right direction? Are you absolutely sure you have the machine set up right. What radio and gyro are you using?
Get some local help from an experienced pilot. That's the best way to solve this problem.
TM