If you manually F5 both sides of the lister a few times does that trigger the problem? If so, does refreshing either side trigger it or only one of them?
It could be that there's a file in the local side which is sometimes causing problems when that directory is read -- as per this FAQ -- and FTP isn't really involved except that it triggers the directory to be re-read.
Another thing to try: When the problem occurs, load Process Explorer, right-click dopus.exe in its list, open Properties and click on the Threads tab. Sort by CPU and you should be able to see which thread(s) is/are using all the CPU. Select the offending thread and click Stack, then post a screenshot of what you see.
That may indicate which component is causing the problem. (e.g. If it's due to a 3rd party DLL that is loaded into Opus which you have but we don't.)