Hi Robert and welcome to WF. I never had one of those heli's but I can throw out an idea or 2, I guess.
Does it use 1 motor or 2 for the coaxial rotors? (I would assume 2, if it has yaw control.) If it's 2, inspect that the motors turn free. Just a hair wrapped on the shaft can mess things up at these sizes. Maybe even try to swap motors if possible.
Stupid question, how about yaw trim on the transmitter?
I don't know if this one has it, but one of the hobby-grade co-ax helis I have has a yaw sensitivity adjustment on it's circuit board.
Thinking about it, this might not be a mechanical problem. When first spinning up, the upper rotor may not spool up as fast since it has that flybar with weights to sling around as well as the rotor. (Did you check the flybar is not bent and still has it's weights?)
Just throwing some ideas out there. Are there spares available for that one? There are a lot of toy-grade helis out there, and unfortunately, they are kinda throwaway items when they malfunction. You have to step up to the hobby grade stuff at some point in order to be able to repair it and keep it flying. If the bug has bitten, maybe it's that time?
Hope something works out for you. Good luck and let us know how it goes.