First, don't worry: when you 'edit' in Lightroom, you are simply creating a recipe for the program to follow when you tell it to 'Export' or create the image. Until you click 'Export' there is no image. Therefore, the pixelation you see on screen is NOT your image, but is the rendering in memory that LR is doing to show you what the final WILL look like. Don't panic.
I am assuming that you are using RAW images. In this case, LR initially uses the embedded JPEG file to view the image. In order for LR to show you a quality image, it must actually do a RAW conversion under the covers to give you a reasonable image to work with. Until it does, what you get is the embedded JPEG, which is really designed to be a thumbnail. So, until LR renders the preview, you will get a pixelated image.
When you 'Import' into LR, you have the choice to ask LR to render a preview, and what kind of preview. The more detailed, the longer it takes. So standard previews take less time that 1:1. For most, standard previews render quickly, but 1:1 wont render until you try to zoom in. You can ask LR to render all 1:1 immediately if you like.
Also, whenever you make a change in the Develop module, LR updates the preview. However, I believe it uses a different cache: Camera Raw Cache. This one takes the catalog preview image and stores it in the Camera Raw cache and then updates this as you change settings in the Develop module.
I believe one or more of your caches are corrupted. First, try the Develop Module cache, by going into LR Preferences>File Handling. There is a section called "Camera RAW Cache settings". Click the button labeled "Purge Cache".
If this does not work, you can quit LR, navigate to where your LR catalog is stored, and you will find a file called "yourcatalogname Preview.Irdata". Delete this file and restart LR. Then in LR, you can select Library>Previews>Render standard previews, and Library>Previews>Render 1:1 previews.
