What if they didn't know the answer ... is that the kind of answers you're looking for??

Here's some help. Learn what it takes to control YAW on a heli. Then start figuring out whether your heli is lacking in any of those areas.

YAW is controlled by sufficient tail rotor authority to control/manage YAW.

Tail rotor authority is derived from rotor RPM, blade size (and correct installation), and blade pitch.

RPM comes from the drive system ... it has to work. For starters, main rotor headspeed has to be sufficient to produce sufficient tail rotor authority (RPM). Is your headspeed sufficient?? Then, as others has mentioned, check for drive train slippage (loose belt, slipping pulleys, etc). This will suck RPMs from from the tail rotor and it seems at times to be intermittant or inconsistant.

Pitch: Determined by mechanical setup/trim, rudder stick position, gyro setup, servo. As you can see, there's a magnitude of things that can go wrong here. If the "uncontrollableness" is always there then it could be in this area. If it's intermittant, then I'd check in the drive train area ... but wouldn't count out a failure in this area.

Blade size / installation: You said you crashed ... right? Many times beginners replace broken parts incorrectly (blades on backwards, rotor spinning backwards, blade gripes backwards, etc). Didn't you say you replaced a tail grip?? Go through this ... ensure this is correct.

As you can see ... this "info" can be applied to practically all helis with a tail rotor. It's because they all have to control YAW in some fashion our another.