I'm guessing the shots where you tighten your grip are outside your comfort zone, thus causing the breakdown in your mechanics. Progressive practice helps with this.
Set up the shots where you tend to tighten your grip, but make them very easy. OB close to the pocket, CB close to the OB. Shoot that over and over to get a feel for what a totally relaxed grip feels like. Because the shot is easy, your grip will stay relaxed throughout the stroke because there's no fear of missing and no desire to "muscle" or steer the cue. Just a pure, relaxed swing. Pay very close attention to how it feels--especially in comparison to your normal tense stroke on difficult shots.
Start increasing the distance slowly until you feel the tension creep back in. Then work on relaxing your grip at that distance. Rinse and repeat with increasing distance/difficulty.