ITunes :: Can't Move Progress Bar Beyond A Certain Point

Strange things happening with the progress bar when playing mp3 files in iTunes (v 10.6.1 (7)) on MacBook Pro with 10.6.8.  The great majority of the tracks I've added since Feb 28th have the following problem:  They play fine all the way through the whole track, but if I try to move the progress bar position to somewhere near the end, it will only go to a certain point.  For example, I have a track that is 7:39 long.  If I click the progress bar to 0:15, it goes to 0:15.  If I click to 0:45, it goes to 0:45.  But If I click to 1:15, it goes to 0:55.  In fact, If I click anywhere after 0:55, it will go to 0:55 and play from there.  i cannot skip to anywhere beyond 0:55.  If I play from the beginning, and use the fastforward button instead, I can FF until 0:55, and then it hits a wall and won't go beyond.  As soon as I let go, it will continue onwards past 0:55 to the end. This is happening on a variety of tracks from completely different sources.   

Some points: 

- All the tracks that have this problem in iTunes do not have this problem on my Touch.  I can jump around just fine there.

- The time to which it jumps is different for each track, and can be anywhere in the song.

- None of the tracks imported before 2/28/12 have this trouble, that I've found

- Happens to VBR and CBR equally, and bit rate is not a factor

- Track length is not a factor

- Start and end times in the options are set to 0 and track end time.

- The few tracks that do let me scroll to anywhere say "Encoded with: iTunes 10.x.x"

BUT, some that are encoded with iTunes are also not letting me skip around.  Still, this might be something?  A problem with LAME encoding?

Info:
MacBook Pro, Mac OS X (10.6.8)