check the cg. sounds tail heavy.
Otherwise, could be other mechanical issues. I have chased my tail on a few models and found these problems as examples:
Upper Jesus bolt was stripped. Head could not be properly clamped onto mains shaft, although it appeared tight on the bench. In flight, the head could move, thus constantly changing disk angles, phasing, pitch ranges in flight as head loading changed.
Generator coupler causing enough vibrations to make the gyro unhappy and inconsistant.
Loose ball link into the main blade grip.
FBL pushrods too thin/soft.
FBL ball links not screwed onto threaded pushrods enough. This allowed the links to flex on the end of the rods too much.
Loose main shaft bearing blocks.
Just to name a few.
Enjoy things that money can buy IF you don't lose the things money can't buy.