First, check your steering end points. This you can do with the "travel" setting on your TX. Whatever you're running up front may be the problem. If the TB03 doesn't have CVD's, the dogbone/cup arrangement is where it may be coming from. CVD's also have a limit to how far they can deflect, so checking and reducing slightly, your settings might help.
Then check the ENTIRE drivetrain, from the motor outward. there may be something impinging on something else somewhere. The TB03 is very tight, in regard to how electronics are arranged, which might be part of why you can't see anything right away.

Whatever you do, do it systematically. Check each subassembly carefully. You don't specify where exactly the problem might be, for not knowing, so go from front to back, and review the manual while you do this. You may have assembled something incorrectly. It happens.