Had something happen like this on a 206b3. First thought I was imagining things as the effect was of such a short duration. Especially on transition to the hover and heavy the hydraulics would become "transparent" and loads on the cyclic and collective made things exciting. You could pick up fly a circuit or to another airport and nil probs, all hyd checks would be without issue. Make a faster than normal app or be heavy and the fun would start.
Engineers traced to a dirty hyd filter that would intimitantly block/unblock meaning the bypass would open/close and the hydraulics would work/not. One of those wonderful faults that our engineers had never seen before.
cheers