Not sure if it's the same issue I had recently with iTerm2 periodically going black, but consider disabling the Core Text renderer with the following command:
$ defaults write org.vim.MacVim MMUseCGLayerAlways -bool YES
This was the developers' suggestion and it solved my issue. There's an informative discussion of the root causes here: https://github.com/macvim-dev/macvim/issues/557.
