You really have to crank your buffers when working with melodyne. I think it was yelling at me to have a min of 1056 or something (which is very high on my machine). Like literally a pop up came up from Melodyne and told me to raise my buffers. I have a pretty fast machine too.
The other thing is I've seen it mentioned it works better doing small hunks of a clip at a time instead of a whole clip that spans the entire song. Essentially you do a range select of the are you want to work on on the clip. Do your edits. Render it then move on to the next section. Once I raised my buffers though I did not have that problem and was able to correct a whole clip of about 3+ minutes.
If that helps, cool. If not maybe something else is up.
Cheers.
PS: I'm still on X3... so maybe there is a problem with Melodyne. I believe there was a patch released that fixed a melodyne problem with Platinum so maybe that's what's up.