This could be the reason your golf game can be so frustrating sometimes. When you lose your posture on your back swing or stand up, it makes you have to be even that much more coordinated on the way back down. Timing is everything in the swing and if you are going up and down in your swing that timing WILL be off.
The number one reason people lose their posture is because of tightness in shoulder, lats and weakness in the upper back. If you can raise your arms over your head without a golf club, good luck doing while holding on to one.
I made a video explaining this process and how you can combat it with simple exercises in the gym or at home.