All good advice. First off, check the main gear alignment as advised above. Check also for a dragging wheel on an axle. Second, up elevator helps immensely and is proper technique with models as well as full scale.

An issue you may have is that the tailwheel gear springs are too soft or too loose. At low speeds the tailwheel can turn, but at higher speeds the friction of the tailwheel on the ground is stronger than the springs. When you input rudder, the springs just stretch without turning the tailwheel. Then when you slow down, the friction decreases and then tailwheel can turn.

Bryan