Unrhetorical answer;): Hmmm... Steam... That's unfortunate. Uhm... Go to the game directory, copy the entire folder, paste it somewhere else should do it, and launch by doubleclicking ksp.exe, that should do the trick. But does the problem still occur when using only vanilla parts? When something stinky hits the fan, and the cause is unknown, reduce the problem to the core, so we can fix it. Something you should take a look at is to make sure that parts aren't clipping into each other. Even if it isn't visible, the (invisible) collision boxes might overlap, causing annoying problems like decouplers not detaching correctly, or parts exploding randomly. So when you attach parts keep clearance from other parts, and pay extra attention when attachment nodes are close to each other, like with decouplers, beams, panels, radial elements, etc. The first thing I would look at in your case, would be a decoupler clipping into the engines. Let us know when you've found something. I look forward to hearing how it went.