Dutch Roll
Dutch roll, in full scale planes, usually means a slight yaw-roll movement at cruise speed. It is very common on swept wing planes. The plane yaws slightly, picks up roll due to the yaw, then yaws back and rolls back. Inside the plane, it feels like you are rocking around.
The Boeing 707/USAF KC-135 Dutch rolled a lot until they installed a yaw damper on top the fin. If you look at the early 707s and KC-135s, you will notice a shorter fin. Later production planes had a yaw damper located in a vertically extended fin. The Air Force planes were retrofitted.
With yaw damper on, the plane was rock steady, but if the damper had problems, it could be all over the place. I reacall refeuling an F-4E during the Vietnam war on a 135 with a bad yaw damper. I was leading the flight and my roommate was on my wing. I hooked up to the tanker and was all over the sky trying to stay on the boom. All the time Mick was giving me this ration about how I couldn't fly. The F-4 was normally real stable on the boom, especially with a full load of bombs. After I topped off, he hooked up and had the same trouble. I radioed the tanker pilot and asked what was going on. "We had a yaw damper failure and are flying manually," was the answer. Made a big difference.
As for the B-70, you might have some yaw instability due to the closeness of the fin-rudder to the CG. Scaling a plane down normally results in the tail being too small for good RC flying. In addition, a delta configuration has most of the wing area located near the center so you get very little natural damping in roll. The deltas I have flown, except for the Diamond Dust, have tended to be roll sensitive around center. This could make it sporty on a test flight even if you are a really good pilot. Once you get it sorted out, exponential helps with the center sensitivity. Good centering servos also make it easier.