When riding in the pack your attention is on keeping position so it's not surprising that your form starts to change. It sounds like you're doing the right thing when it counts. I do the same thing as you do when I'm on the flats, especially if I'm pedaling at my usual 90 rpm and I'm not working very hard at 16-18mph . My mind will start to wander and I'll forget about pedaling circles. If I need to generate more power then I have to focus and then I start making an effort to get back in form. I find that shifting over to the big chainring and going at a higher gear that I normally ride in forces me to focus on my form since I have work harder to maintain the same cadence.