my images that I downloaded are either all split up (picasso like) or have colourful streaks
Two troubleshooting steps:
1. Uncheck the option Preferences > Performance > Use Graphics Processor. Then update your graphics driver directly from the manufacturer's site: Adobe Lightroom GPU Troubleshooting and FAQ
2. If you are on Windows with NVIDIA graphics hardware, you may have encountered a bug in the latest NVIDIA drivers and need to roll back: Image previews and thumbnails look tiled or broken on Windows
In August my images that I downloaded are either all split up (picasso like) or have colourful streaks. this only happens to RAW images not jpeg.
The issue of a photo being all split up should be addressed by #2 in the previous reply.
Can you show us a screen capture of the "colourful streaks"?
I use a MacBook Pro.