I have the latest SF Pro, but this can apply to earlier versions as well. After selecting a portion of data, I'll frequently cut it to the clipboard with Ctrl+X. Then I bring up the Mix Paste dialog and hit Preview to listen to what it will sound like before actually pasting it into the sound file.
Could there be any possible delay or other difference between using the preview feature and actually making the final paste into the sound file?
Also, occasionally when I cut to the clipboard and preview the mix paste it will freeze for a few seconds after doing a preview and I can't stop the preview orr do anything during this time.