Make sure Clear previous results is turned on in the Find panel.
If you aren't already on the latest version, update to that first.
If you're still seeing the problem with the latest version, see if you can narrow it down to when a particular file, or type of file, appears in the Find results. If you can then the troubleshooting steps in the Crash, exit or high CPU when viewing a certain directory guide should help track down what is crashing. (Often a video codec or viewer plugin.) Turning off any columns which may cause Opus to inspect the files for media metadata may also help if the problem is caused by certain files or filetypes.