Pretty normal MG behavior, actually. Just keep in mind that, when putting a build together, you need to refresh the page on a semi-routine basis to avoid these "jumpy modules". Also, when you have the "slide-under" problem, just move all of the jinky-acting modules to outside the cab, then put them back in carefully so that you're always getting everything properly side-by-side. Usually that, plus a refresh, fixes much of that nonsense.
The other important thing is that you have to use that refresh on the SCREENSHOT once you've completed things. The screenshot is very important if you're aiming to show your build, as that's what the Forum uses for the builds you see being discussed here. So when you're done, pull down the screenshot (from the 'Views' pulldown) and refresh it. At that point, it should perfectly match the build page. If not, refresh that screenshot again.