A/B Roll and Double Exposure (and some other effects) are very demanding processes.
Sometimes, A/B Roll can swallow up too much of the unit's CPU processing power, resulting in bad audio or stuttered video. If you have a section that exhibits this problem, finalize your edits to that section, then employ the Scenemaking function to create new Scenemaking material of that section.
Be sure to do a Scenemaking of the entire section that has A/B Roll or Double Exposure. Once you have the new Scenemaking material, replace the A/B Roll effect material with the newly-rendered Scenemaking material. The CPU can now easily process the video and audio.