Images in Lightroom go from sharp to blurry

I am using Lightroom to edit photos and find that when I zoom in on photos they often start sharp, them become blurry. Sometimes they go back to being sharp with no intervention, but sometimes I have to zoom back out then back in for the problem to resolve. This is very annoying since I am working with thousands of images and trying to choose the sharpest ones. A delay with every photo costs a lot of time.