I think you're biggest problem may be that you are actually trying to do too much behind the line. Bowling a sport where less will more often than not yield more for you.
What I noticed is that the difference between your well balanced and timed up shots and the not so good ones were two things.
1) You are losing your hand position too early and as a result you may or may not be aware of it, but you are trying to tear the cover off of the ball to get it to hook for you.
2)It looks like you are charging the line some and need to slow down your foot walk. Most of the time this happens from some late timing, eagerness to rip the hell out of the ball, or thinking that you need to do more to get the reaction you want.
If you've ever bowled in a more traditional style of play before. I'd recommend instituting some smoothness and balance from that especially at the bottom of the swing into your style of play. I find that when I roll with your style it makes it much easier on the body.
Hope that helps,
The Demolition Man
Be like water making its way through cracks. Don't be assertive, but adjust to the object, and you shall find a way around or through it. If nothing within you stays rigid, outward things will disclose themselves.