The gyro may be too close to the ESC. On a non-micro heli, the gyro should be at least four inches away from the ESC.
Tail belt may be twisted around. Removing the tail rotor assembly from the tail boom and make sure the tail drive belt isn't twisted.
The gyro sensitivity may be set too high. Try lowering the gyro sensitivity (channel 5) until the wagging stops. A Logo 20 sized helicopter usually requires about 50% in AVCS mode (75% in ATV mode) for good performance. An ECO 8 usually requires the sensitivity to be about 30% in AVCS mode (which is 65% in ATV mode) to stop hunting. Smaller helis will require even lower sensitivity.
The main gear or other gear may be missing a tooth. Visually inspect the teeth of all gears and verify they are all intact.
Tail belt may be loose. The belt should be firm but not too tight.
Excessive play in the tail pitch control system can cause tail wag. Make sure there are no loose screws/ball links/clevises in the tail pitch control system.(A little play is normal, but not too much)
The tail rotor shaft may be dirty and the pitch slider may not be sliding smoothly on the shaft.
This is fairly easy to fix. Remove the tail rotor shaft and throughly remove all gunk using some tissue paper. A cotton swab stick is just under 2mm in diameter, so cut a cotton swab in half and use the two stick sides to clean the inside of the tail pitch slider.
Do NOT apply any oil to the tail pitch slider because it will become dirty extremely quickly. If lubrication is needed then use powdered graphite instead.
The tail blade grip bearings may be dirty and/or damaged, or loctited to screw.
The tail blade grip screw may be bent.
Make sure the servo is mounted securely to the frame or tail boom mount. Check for loose or stripped screws.
Make sure there are not rubber servo grommets used for the tail servo mounting. This can cause wag.
Try replacing the tail servo. The potentiometer on some servos will wear quickly with constant movement, and this can cause wag.
Check if the gyro is mounted securely to the frame. If the gyro is not mounted firmly with servo tape, then it can wobble back and forth which can cause wag.
The thick foam gyro tape supplied with the CSM LW200 has a very bad reputation for causing problems. The tape will tear across the middle of the foam core and cause the gyro to vibrate which causes tail wag.
This can be replaced with a thinner foam tape such as 3M Scotch double-sided foam tape part number MMM114.
Tail servo may be missing teeth on a gear from previous crash damage.
If using a GWS IPS motor with a 3x2 propeller for a tail, then the prop may be too large. Try trimming the propeller by a little to check if the tail wag decreases.
This could be caused by radio glitches. Make sure your motor leads are short and routed away from the receiver.
The radio receiver crystal may have been damaged in a previous crash. Try replacing the crystal or entire receiver.
The one-way bearing in the autorotation hub may be loose.
Tail servo responds properly when tail swings one direction but tail servo fails to respond in other direction.
The gyro may be damaged.
Tail holds fine at the beginning of a flight, but starts to drift as the battery discharges. This is normal for a yaw rate gyro. Heading hold gyros do not have this problem.
The heli tail runs even at low throttle when using a heading hold gyro and a tail motor. This is normal. The heading hold gyro does not output zero throttle unless it receives a full left rudder stick. This is why you need to hold full left rudder to initialize the tail ESC.
Tail servo slowly moves in one direction (creeps) when heli is on the ground with heading hold gyro. This is normal. Don't worry about it.
Tail servo travel is unequal on both sides of the neutral point with heading hold gyro. This is normal. You should set the servo travel in the non-HH mode and the heading hold mode will work automatically.
Helicopter has a tail motor, and has insufficient tail thrust to counter main rotor torque. Tail prop may be mounted backwards. Examine the prop airfoil - the prop should be mounted so the airfoil will gradually increase pitch as the air moves past the rotor.