Which encoder was used to create these files?
Anyway, to rule out file problem, please install foobar2000, load all files from that folder into it, select all in playlist, right click then select Utilities / Verify integrity. Then, if there are errors, you can try Utilities / Fix VBR MP3 header, or Rebuilding entire stream.
I am guessing that some of the files came from one-file mp3 and cue file, and then cut to individual files without reencoding by some brute force tool, but without rewriting complete file stream. But I could be wrong.