I had a bad gyro once, simple telebee like gyro that caused the symptoms you described but mine was pirouetting like crazy upon throttle up.

One way to test on the ground is to power everything on, put the throttle hold on and grab the tail gear in your hand and push the tail away from you ... the gyro should tell the tail to compensate and push the tail slider in the opposite direction (towards you) as if to turn the nose of the heli right. That's one easy way to see if the gyro is operating correctly.

Tail wag can be overcome by decreasing gain on the gyro channel, usually done on the gain channel on your TX or on the gyro itself if you do not have that option.