btw, raising the common gain to 1.40
and configuring gain on tx at around 90-95
finally led me to some oscillation
but only in idle2 at 2300 rpm
and tail is still not so precise
If running the governor, the gain could be too high or too low for response gain.
Your tail thrust bearings could be worn out.
The tail servo could be going bad.
How much collective pitch are you running?
What is piro consistency set to? I would start off around 155 and only turn up main tail gain as much as possible first. If it's already low or default then check the things above.