I believe you are having a problem during the AMBUSH portion of the rotation. Remember if you do not have enough energy use this time to cast Rifle Shot / Shatter Shot. I honestly rarely use AMBUSH because I would rather do Lazed Snipe / RS / recast my Shatter Shot debuff. Also I made a change in the rotation. To easier sustain it (practicing on the dummy) you start with:
IP - CG - Rifle Shot - CD - Rifle Shot if needed - CULL - SoS - Lazed Snipe / Shatter Shot - Rifle Shot - Cull. Repeat.
Use Ambush in place of Lazed Snipe or Shatter Shot if you have enough time.
It is ok to cast Cull if you have 82-84 energy. If you do not have 82-84 energy you should use Rifle Shot.