I am rendering "standard" previews on import (default is minimum I believe) and I selected "medium" previews for DNG JPG preview in the preferences. I do convert to DNG on import.
I believe default is "embedded and sidecar." I asked about 1:1 previews because LR does this to me sometimes as well. As soon as the 1:1 previews are loaded, they photos look great.
Give it a try and see what happens.
Just wondering, why are you converting to DNG?

