I'd have to double check videos on heli freak but I think that one sets up the same as my JR G500T. If I'm right double check your mechanical setup. Make sure it's equal on both sides. then zero all your trims & subtrims, then go to your gyro menu on the Dx6i. (same as mine) turn gain all the way up to 100% and watch for any drift in the tail slider. if it drifts use sub trim to dial out the drift.
Then lower gain to about 65% & Check for drift in the tail when you first plug it in & after everything has intalized. if any drift, again use subtrim to dial it out. Then try flying again. That's what I had to do with my JRG500T. The older gyros set up differant from the new ones, requiers more fideling to get them right. Mine olds good now but I had the same issues. Going crazy sounds like gain is to high in idle up