I've been having the same problem with audio books and other long files on my m240. I can usually fast forward 10 minutes or so, but above that I risk jumping to another track and having to start over again. Also, when I pause a track in the middle and allow my player to shut off, when I turn it back on it says it is playing 20 minutes into the track or at whichever point I left off, but it actually starts the track over again from the beginning. I've been spending a lot of time sitting at my computer finishing the track so I can start the next one on my player. I guess that's what I get from a player I paid $5 for on Woot, right?
The audio books I've been listening to come from my local library via Overdrive Media Console. I'd use my Sansa e280 but the files from Overdrive don't even show up on the player. It's a pain in the butt.