How large are the files? If they are really huge or very long, the Sansa probably has to stop eventually and decode some more.
If you’re on WIndows, right-click on a problem file and click on Properties. What is the size (in MB)? Bitrate probably shows too.
Some people throw all kinds of junk into the tags–the electronic labels in the files. You can get the free http://www.mp3tag.de/en/ to take a closer look at them. (Download the file that has a version number and setup.exe, not other junk on the page.)
During installation, allow it to add itself to context menus. After installation, go to Tools/Options/Tags/Mpeg and make Write ID3v2.3 ISO-8859-1.
Right-click on a bad folder and highlight a file. If there are huge Comments, you can go to the menu at the comments box and make it <Blank>. You can highlight all the files and blank out the Comments at once.
When you highlight a song, do you get an image displayed in mp3tag (lower left)? Go to View/Extended Tags and click the little floppy disc icon to save the image in the folder (instead of in the file itself) and then click the X to get it out of the file. Against, you can do that with all the songs in a folder at once.
Those are two things that can trip up the Sansa.
Mp3 can be any bitrate up to 320 kbps. Mp3tag will also show you the bitrate. But the Sansa can play just about any mp3. The problem is the amount of information it has to digest. That’s why the suggestion above is to make big files smaller by converting them to a lower bitrate.
You need lots of bits to make music sound good because there are low notes, high notes, sustained notes, percussive notes. A voice (like an audiobook) can sound good with fewer bits. But if you’re playing songs, then there’s probably some kind of clutter in the tags.