ORIGINAL: elmote

Its not your curves, as I assume you are hovering only and not throwing a lot of pitch chance in. If the heli is holding well when you remember to use HH but constantly wants to piro (spin) in rate (normal) mode you dont have it setup right mechanically. In rate mode the heli when turn in to the wind (weather vane) but should NOT piro with pitch/throttle changes (it may swing just a bit, but should be easy to compensate for). If you are having to hold in a constant correction to keep the bird from spinning even in an otherwise stable hover, you need to make adj. to the links. ALLWAYS setup the linkage in rate mode toget the rudder centered. Try this-

Training gear with free-spinning balls.
IN RATE mode, get the heli light on the gear, but not airbourn.
A spin during spool-up is OK, but when full speed is reached, the tail should react to any spin and stop it. It wont return to specific heading, its in RATE mode, but it should be steady if you arnt making collective changes.
If heli still is spinning, look at which direction the NOSE is turning. Let's say it is turning to the left (CCW), and you have to give constant right stick to stop it.
Throttle down and stop blades, engine running, TH on.
Look at tail servo. Is the horn at right angles to the push rod? It must be if the stick is centered in rate mode.
Now, put in LEFT stick, commanding a CCW turn (same direction the heli was spinning!).
Does the servo PUSH the linkage, or PULL it to transmit this command to the tail?
Whichever, it is doing this ALL THE TIME. If it is pushing, the linkage is too long. Pop the links and turn them in 5 turns each to shorten. Opposite if the servo is pulling.
Repeat the ground-hover test. Piro tendancy should be reduced, eliminated, or even reversed.
Make further adj. as needed until heli will sit light on the gear with no spin in rate mode.
Now you have a centered linkage. In rate mode heli should sit weathervaned into the wind in hover with only slight self-limiting swings in response to your inputs, and weathervane with direction of flight in FF. Flipped to HH, should remain locked on heading regardless of inputs/wind/flight direction. If not, you will have to play with other parameters like gain, but at least the basic centered linkage is good. Without that, your chasing the end of the rainbow.

Elmo Te