i have found that most interactions occur near the limits of servo/linkage travel. If your problem is right when taking off, its probably what bum said, simply the normal drift caused by the tail rotor at low altitudes. If you get your heli in a stable hover at about eye level and dont touch the cyclic and simply watch where it goes, trim opposite and at least your hover should be somewhat stable. Interactions are a bit trickier because fixing one thing usually causes something else to screw up, basically you most likely wont be using the extreme limits of travel anyways so get it to fly solid through 90% of the travel and by the time youre ready to tackle that last 10% it wont really be much of a problem anyways.