Mail for Mac: Delays and blank messages

Mac Mail is causing me two huge problems

1 - Mail comes through in batches and often with delays - I have changed the settings for check new messages from Automatic to Every Minute but this has not improved it

2 - Often messages are downloaded and are blank or get stuck showing status 'downloading' - I find I am having to rely on my iPhone and/or iPad to check emails because MacMail is no longer reliable