Ground effect is noticeable within one half wingspan of the ground (as mentioned earlier), not merely 1/2 the chord.

I corroborate some of what has already been written. See if this makes any sense to you:

Consider the following, it's a common cause of sudden left turns with taildraggers: As the tail is coming up on takeoff, gyroscopic precession causes the nose to yaw noticeably left. This is not a theory: it's a physical fact. The only way to properly correct for this is to anticipate it and apply a bit more right rudder as the tail is coming up, then decrease it once the tail is up and it's rolling along the ground on only the mains. Otherwise, if you wait to see the yaw you will yaw harder left than you mean to and/or over-correct every time. This agrees with your comment that if you get the rudder in just right, you can take off instead of ground looping. It also agrees with your statement that you have to use reduced power for takeoff. Precession is a strong force and shows up well on short-coupled biplanes with small rudders. This assumes you are allowing the tail to come up on the ground and are not taking off in a three-point attitude. You can reduce the precession-induced yaw moment by using a lighter weight propeller or reducing RPM (by going to higher pitch) or reducing propeller diameter without increasing propeller mass or RPM.

As for the landings, it's likely that as the airplane slows, the rudder loses it's ability to steer the airplane. That's why it's important to have a tailwheel rather than a skid, and gradually feed in up elevator as the airplane slows, to make the tailwheel steering make up for the loss of yaw moment from the rudder.

It seems unlikely to me that ground effect has anything to do with it - primarily all that does is reduce induced drag. Prop slipstream and tailwheel steering are life when it comes to directional control on the ground. Also remember, that almost all full-scale airplanes have differential braking to help with ground steering at low speeds when the rudder is ineffective. Models have to make up for this lack of differential braking somehow, usually by using a larger than scale rudder and extra-careful use of elevator to maintain tailwheel steering at slow airspeeds.