>Everything seems normal. I do notice that the "Not responding" message from Windows seems to occur while the program is doing some data processing. It happened today while indexing the files.

The app is receiving an "are you alive?" message from Windows every 4-5 seconds, and if it's in any kind of wait state, it will respond. If it's busy for longer than that, it won't, and will be marked with "not responding".

So, try to do a Doevents Force often enough, see if that helps.