1) Did you try to play these files from that flash drive on another device? I assume they were created on PC by ripping CD. Try different PC. I suspect the conversion to WMA may have been flawed or generated files "slightly incompatible" with 730N
2) Try converting original CD to MP3 or get some MP3 that sound good on your computer and copy them to 730N.
I personally have 430N and never tried WMA. No problems with MP3 at all
I have no issues (730N here), but I've been using MP3 files and not WMA lossless. I might try a MP3 just to see what happens. If it works fine, it's the format causing the issues. If it still happens, there's probably something wrong with the unit.