Do NOT re-install the OS unless absolutely necessary. But, yes, do it if you only have a few programs to re-install.

Now work by the process of elimination. Your processor might be overloaded or another program might be interfering. This is what I do...see if it works for you.

Turn ALL programs off. Go into "safe" mode if you can play a MIDI file there. If you're not in safe mode it's OK too. Make sure any running programs are turned off (except Explorer). Go to Ctrl+Alt+Delete to turn everything off. Only your MIDI player should be running.

Now play the questionable file (or any music file you're having trouble with). If it plays enough times with no errors then you know the file is good, and the problem is another program affecting the playing of it. If you STILL have the problem, then you've eliminated other programs affecting it...or...processor overload.

I assume you're doing virus checks regularly. You might have a bug in there.