This was happening while I was adding furniture (windows in this case) without changing anything else. I have tried using Integrated Graphics on my motherboard (Intel HD Graphics 4600) and the problem went away. I suspect my graphics card or its driver was the culprit even though the driver was the latest available and the graphics card was working perfectly with other software. For now, I will continue using Integrated Graphics but I'm afraid I'll lose performance when my model gets bigger. We'll see. Thanks for your replies.