if you had to lower it that much, I would say mechanical too.....mine is at 75 and it doesn't wag at all.
things to check:
Tail rotor hub (not bent)
tail blade grip radial bearings and thrust bearings. be sure the thrust bearings are good and lubed up.
output shaft trueness.
bell crank not binding, or loose
tail control rod straight and not binding. be sure to use the shrink wrap on the rod where it goes through the guides.
tail blades balance.
I run sealed bearings in my tail grips instead of the open bearings. I know you cannot lube them as easily, but I feel like they last a bit longer (provided you don't crash
)
my inconsistent tail rotational speed I mean it will literally "whip" around. so the tail rotation speed will increase in half of the pirouette than slow down on the other half. I talked to a bud of mine that is very keen on the vbar, and he told me to re-do the optimizer on the tail and it should fix it.