Hello Dave

This sounds like a radio/stick deflection issue more than the Tamiya electronics. I'm musing here but it sounds like the left and right sticking is acting like a trim is not zero'd properly. Maybe a negative shift for extreme deflection?

The fact that you can gently turn from a forward (or reverse position supposedly) may indicate a range near the extreme left or right that reverses the motor direction.

Can this issue be duplicated from a reverse steer direction? Have you tried another radio? Is the crystal socket secure?

Check to see if there is a small range in the extreme left and right that reverses the direction desired.