Either your blades are off balance or your blades are a bit too tight causing them to align only at the highest headspeed.

Normally, some unstability during spool is normal as the blades auto-center. If it continues even into a hover, your blades either need to be balance, or are too tight.

There is also other problems that can cause this, but those are the most common. Other problems can be blade tracking, flybar centering, main spindle or feathering shaft being bent, etc.