My 500 bobbed up and down once or twice due to low headspeed. I finally figured out that it was a bad cell in one of the packs that I had wired together. I was using two 3s 2170 packs until my dedicated 6s packs came in. I pull that bad pack out of my rotation and never had another problem.

Have you used a tach to measure your headspeed? You can't solely rely on your throttle curve to estimate headspeed.

Like HeliSmith stated- "Can't get the tail to hold until you can work out the low head speed problem." Get your headspeed up and see if that solves both of your problems.