Steelfury

Have you tried throttling back a bit when you reach the upper atmosphere? I find with the big rockets using mainsails that as they empty they cause a lot of instability if on full throttle. Also, attaching fuel tanks radially with smaller engines on them and then dumping those once they're empty seems to work well.

While there may be issues with really complex multi-stage spaceships at the moment, I've loved this game from the moment I bought it, being able to design a spaceship or a spaceplane and then fly it is an incredible experience, and the level of difficulty makes it feel like a great accomplishment when you do manage to land it on your destination. As to the lag: have you tried using poodle engines on a second stage with mainsails on the first stage? As long as you add a couple of struts and go steady on the throttle with the mainsail engines I find I can get some pretty large ships into orbit without the dreaded framerate issues associated with using tons of smaller components.

From those pics it looks like your solid fuel boosters are wobbling all over the place, try sticking one or two struts on them attached to the body of the rocket