I am using Media composer 8.4.5 on a Macbook Pro, and find that my timeline view loses the ability to display the waveform after increasingly short periods of time, and never gets it back. It's very frustrating: I'll see the waveforms suddenly disappear while editing, I make sure that I'm still displaying waveforms in that timeline view, I'll switch to another view that shows waveforms and see that it's still working, then go back to my original view and the waveforms are still gone, even after a full composer restart, toggline all settings, changing sequence, etc.
I have a very specific view with Aud 1-4 big where I keep losing it, and it takes a few minutes to recreate the view each time this happens. And now I find it happens less than a minute after resuming editing with the new view.
Are you using timeline view of waveforms or the track based ones available on each track? You may have a waveform cache issue - you could try deleting the contents of that folder in the project folder and letting them rebuild and see what happens.