Apple mail not loading emails correctly.

I have macbook pro, retina, late 2013, 2.4 GHz i5, 8gb ddr3, running yosemite 10.10.5. A couple of months ago, apple mail started failing to load emails correctly. Sometimes only a single email listing would load (left screen) with no previews showing on right half of screen. Sometimes email listing would show many emails, but the preview on right half would be blank whether an email was selected or not. I would have to quit, then force quit the app to close it, and try again, sometimes several attempts to get it to work.

I have read a couple of suggestions about refreshing this or doing that... but what the heck, it worked before, why not now, and when will it be fixed? I even dumped a large number of emails, and it didn't help. This is clearly a deviation. I thought the latest upgrade might help, but to no avail.

How to fix, or more importantly, when is Apple going to fix it?