The spindle I mistakenly installed was slightly bent in 2 places (a shallow 'Z' bend). It evidently introduced enough friction into the collective mix to continue bogging down the main motor. When I installed the correct, straight spindle, the motor resumed normal operation.

Also, I checked pitch again & reduced by another degree or so @ the pitch links. Head speed is now brisk w/woodie syms & ascends like a missile @ liftoff if I goose it even a little.

And, last but not least, the little tail motor rears its head one more time. The G90 was definitely doing its deed but tail performance was occasionally erratic & would zip a 90 deg CCW rotation w/o warning. Right rudder input would rotate the model another 90 deg CCW, resulting in a BCPP looking me in the eye & wondering where to go next! I'm not really ready for nose-in yet so would do a quick controlled setdown.

Suspecting sundry weirdness in the tail motor (only about 3 weeks old), I re-installed the original tail motor just for grins. I had replaced it simply as part of routine PM (due diligence, you know?). That cured almost the last of the quirky behavior & am flying it well w/G90 HH about 30%.

The only remaining oddity is, when flying HH, tail rotor spins rapidly (momentarily) when I advance throttle stick from full-off, ground looping the model nearly 90 deg CW. I get around it by starting throttle with some left rudder, then centering & lifting off. At that point, the tail rotor spins up quickly as the G90 attempts to maintain heading. From there it's business as usual.

If that's all I have to worry about now, I'm one happy camper as I practice transitional side-ins.

BTW lwatson your point #4 turned out to be well-taken. Not only did the backward 'landing' do a number on the spindle but evidently did some internal damage to the new tail motor as well. I'll lay in a supply of them; still want to stay mainstream parts-wise until I'm a solid 2D flyer.

zenchal you were on the right track after all w/your tail motor suggestion. Turns out, despite its young age, to have been one of the problems after all. I just never had one fail & was unsure what to look for.