Twitchy tail rotor

I have an Heli-Max AXE CP all electric helicopter.
Recently the tail rotor has become twitchy.  It changes it's RPM often
and instantly.
Twitchy is the best way I can describe it.
This makes it pretty hard to control the heli.
Thee heli has had a few tail strikes on the ground ( enough to break
the rotor and gear teeth)
and a few low altitude (3 or 4 foot) crashes but nothing I would have
considered very bad.

Any idea what might cause this.  I hate to just start replacing things
that aren't bad.
Low transmitter batteries?
Bad ESC.
Bad tail rotor motor?