HI,
I had the same occurence on 2.34 (once only a few months after installation) on Server 2008 R2 64bit.
I poll 1 directory every minute for an FTP upload and delete from source.
The Fling and Windows logs didnt reveal anything. It just seem to stop without reason
I also go upload "OK" in the Fling logs and had files missing on the remote FTP server, scary!
I reinstalled and stable again ever since. Who knows if all the files actually upload, Im sending 1000's a day.
Check your memory to see if you are maxing out? I doubt you wil get any assistance from NCH for old version, or so they lead me to beleive. I get GUI only support (after paying extra for support) for these development type issues; about as useful as a a solar powered torch.
Good luck.